THERE IS A dirty little secret in Washington, one spoken quietly over lunch tables and between friends, but not really shared with the rest of the country, except in code. The secret is a widely felt anxiety that four years of the Reagan administration just may produce genuine disaster -- economic disorder, international chaos, or worse.

This is not a partisan secret. Indeed, it's often easier to pry it out of a Republican than a Democrat, perhaps because Democrats are still terrified by the intensity of their drubbing in 1980, and still awed by the man who gave it to them.

"Let's be honest about what we're dealing with," says one of this city's most thoughtful and influential Republicans. "This (Reagan) government is just a reflection of a mood in the country that said, 'Anything but Carter,' and this was the available alternative. . . . Expecting them to be serious and effective is like asking a California surfer to look beyond the sea and the sand and the sky."

Nasty talk. We like nasty talk in Washington, of course -- always have. The last administration -- Jimmy Carter's -- was also a source of deep anxiety for the capital cognoscenti, who looked with dismay at a bumpkin president who got trapped in minutia and was surrounded by people who couldn't help him find a way out. Now we seem to have traveled to the opposite pole: an administration lost somewhere in its latest All-Purpose Generalization.

This is all by way of introduction to Judge William P. Clark, who typifies much of the Reagan administration. The judge is genial, honest, serious and efficient -- another nice guy in the White House. He also happens to be the first national security adviser since the job was invented who has no experience or prior knowledge of national security issues. He is Ronald Reagan's key assistant on the foreign and defense policies that they both know very little about.

So what do you want, McGeorge Bundy? Walt Rostow? Zbigniew Brzezinski? No, the experts haven't always had the answer, either. Many of them were so determined to reshape the world that their major accomplishment was to mess it up. We haven't yet found a happy medium in this business. But it's unlikely that the best alternative to misbegotten experts is ignorance.

Talking to foreign policy experts about Judge Clark is fun. The judge is a very important man, of course, which ensures some positive comment from the knowledgeable, particularly those waiting for their expertise to be converted into administration policy. But most of the specialists have reservations. They do believe in the value of knowledge and experience. They wonder if a government can do more than cope with the crisis of the moment if so many of its key players have so little of either.

Of course, the experts generally believe that a national security adviser should be one of them: an intellectual who knows the world, a conference-attender and author of op-ed analyses.

A critic who served with Clark at the highest reaches of the State Department when the judge was deputy secretary, for example, says it demonstrated "contempt for the public's business" for Clark even to accept a critical job for which he is so ill-prepared: "Say your neighbor is a plumber -- do you invite him in to do brain surgery on your kid?"

At State, one learns from this source and others, Clark did very little work on the substance of foreign policy. It was a standing joke among the corps of senior-assistants-to-senior-officials that they never knew how Clark was spending his time. Nevertheless he was a popular figure at State, as he seems to be wherever he goes.

The judge wasn't sent to the State Department, of course, because of his standing in the foreign policy community. He was there as assistant to the vicar, to keep an eye on then- Secretary Alexander M. Haig. It was at the White House that Clark was a formidable figure.

Washingtonians who tuned in late to the Reagan-Clark relationship cannot appreciate its closeness or durability. As Lou Cannon makes clear in his intriguing new biography of Reagan, Clark deserves considerable credit for salvaging a Reagan governorship that -- after a year in office -- was in danger of crumbling. Clark moved in as the governor's chief assistant and devised -- among other innovations -- the "minimemo" for explaining all issues to Reagan, no matter how complex.

The minimemo invariably contained four paragraphs: one to state the problem, one for facts, one for discussion, and the last for a recommendation. This and other aspects of Clark's performance so pleased the boss that at one point, when the job was vacant, Reagan wanted to make Clark lieutenant governor of California.

In the first phase of Clark's diplomatic career, he became Haig's most important protector. By all accounts at the State Department and the White House, he came to admire Haig greatly, praising his mastery of foreign affairs and shielding him from the criticisms of others close to Reagan.

But the Reagan foreign policy team did not work well during 1981. The national security adviser, Richard Allen, was a formidable failure at his job, and the president managed to get through his first year in office with only the most cursory glance in the direction of foreign policy. (Clark has conceded this embarassing fact in private conversation.)

When Reagan asked Clark to come into the White House at the beginning of 1982, the judge got an extraordinarily warm reception from the news media, who praised his genial manner and earnest efficiency, and generally ignored the question of his substantive views. This was -- in part at least -- a tribute to Clark's skill at cultivating reporters.

The tall, gentle, unassuming judge who always wears cowboy boots had managed to disarm a press corps that originally ridiculed him for his know-nothing performance at his Senate confirmation hearing. Clark did this with flattery and enticing information.

According to one associate, the judge developed personal relationships with about two dozen reporters and columnists, many of whom believed they were in a tiny circle of favored intimates. He became a reliable source for these reporters, never failing to return their calls, even if that meant telephoning a reporter at home on a Saturday afternoon. He was invarriably polite and helpful, and sometimes served up meaty information, including gossip about clashes between Haig at State, Ed Meese at the White House and Cap Weinberger at the Pentagon. Reporters understandably love sources like this. Such sources rarely get burned.

So Clark got a good press. At the same time, he continued not to have any known views on most national security issues except on the broadest questions. Some of Clark's thinking was revealed in a speech last May billed as a description of the administration's new approach to world affairs. It read like a civics text for 10th graders:

"First of all, the purpose of our strategies should be to preserve our institutions of freedom and democracy, to protect our citizens, to promote their economic well-being and to foster an international order supportive of these institutions and principles."

Such attempts at formal rhetoric are -- perhaps mercifully -- rare. Clark feels that he should not be a prominent spokesman for the administration, that he should remain part of the background music. Therefore he refuses to give on-the-record interviews.

But it can be reported with confidence that he neither denies nor is troubled by his lack of expertise. The judge, it is said, believes he can handle his current job like a judge, listening to various points of view and synthesizing them for the boss, a talent he honed years ago with the minimemos.

If expert background is needed, experts in the government can provide it. A knowledge of history is rarely required, because decisions are made on the basis of current information, usually events of the previous 24 hours.

Despite his judicial attitude, though, Clark does bring a strong personal point of view to his work. He is, as several friends describe him, "a Catholic conservative" who believes in old American values and is staunchly anticommunist. He is a churchgoer whom friends describe as a devout person. Clark is said to feel that his paramount duty is to help the president follow his own instincts in foreign policy.

Clark's attitudes are reinforced by his two principal associates on the National Security Council, Robert C. McFarlane and Thomas C. Reed.

McFarlane, a former aide to Sen. John Tower (R-Tex.), was a principal author of the Republican Party's 1980 platform planks on foreign policy calling for American superiority over the Soviet Union. A retired Marine colonel whom one old friend describes as "smart but narrow," McFarlane is expected to prevent Clark and the president from saying or doing anything foolish.

Reed, a former secretary of the Air Force, is a wealthy Californian who worked for Reagan when he was governor. Clark put Reed on his staff earlier this year despite an embarrassing run-in he had with the Securities and Exchange Commission over a dubious stock deal. Reed is a hardliner who has moved quickly to a position of influence, often briefing Reagan personally.

Reed revealed a good deal about his view of the world in a speech he gave just last week: "We should be proud of America, and we should hope to prevail," he announced.

"These concepts of 'pride' and 'prevailing' certainly are considered novel in present-day Washington," Reed averred. "Not since John Kennedy left the world stage have we dared to dream such dreams. . . .Once the democratic institutions of a nation, after observing the procedures of due process, decide that the nation's vital interests are at stake, there's nothing wrong with winning."

In the light of such insight, it is not surprising that the notion of imposing sanctions on our allies to harass their planned natural-gas deal with the Soviet Union came out of Clark's National Security Council apparat. Europeans of all political persuasions, from Britain's Margaret Thatcher to West Germany's Willy Brandt, tell us that this move has turned into a fiasco, hurting the Western alliance much more than in has hurt the Russians. But Clark is said to feel that it was a good move, one that will be applauded in time.

Eventually, he has told friends, the issue of the Soviet use of "slave labor" to build the pipeline will persuade Europeans that they have made a big mistake.

William Clark is an American phenomenon. There isn't another Western democracy that could put a man of his background at the pinacle of its diplomatic and security apparatus. But then, there isn't another country that could elect a man of Ronald Reagan's background to the presidency, either.

Clark is a product of a uniquely American invention, the American legal community. Most societies use lawyers for specific purposes like dealing with legal matters, but we use lawyers to do almost everything except play third base. Our theory is that a good lawyer can master any subject and deal with it effectively.

But there is a potential weakness in the notion of national security adviser as judge. Judges can only deal with matters brought before them by plaintiffs and defendants. In practical diplomatic terms this means crisis management, not purposeful strategy.

The record of the Reagan administration thus far suggests that it is much better at reacting to problems brought before it (the European and American anti-nuclear movements, the Middle East crisis) than with bold initiatives of its own (Central America, the pipeline sanctions).

Will Washington's inklings of disaster prove accurate? Will Judge Clark and his colleagues really make a mess of this country or the world?

Not necessarily. They are muddling through for the moment, though the problems ahead loom large. Previous intimatithe baons of disaster have rarely materialized. Even Jimmy Carter is already looking better.

The country, arguably, took quite a risk putting a Ronald Reagan in the White House, and Ronald Reagan took quite a risk putting a Bill Clark at his side in such a sensitive position. To a considerable extent we are in the hands of novices -- a disquieting proposition, but one that would be even scarier if the experts had a better record.