Five months after his appointment as President Reagan's national security affairs adviser, William P. Clark has dampened the internal power struggles and policy disagreements that marred much of Reagan administration policy during its first year.

Through a series of directives signed by his old friend, the president, the former California judge has also moved to force sometimes stubborn, turf-conscious Cabinet officers with conflicting views to provide more coherent plans for the years ahead.

The fact that he is faring so well is, in the view of experienced officials throughout the government, remarkable in itself because Clark is the most inexperienced person in foreign policy and security matters to hold the key job at the president's elbow in two decades. Moreover, the White House National Security Council staff that he heads is still viewed by a number of experienced officials as the weakest in many years.

In a sense, says one official, there is a "revolutionary experiment" going on. Clark, despite his lack of experience, is managing to be a powerful force for getting policy choices out of the bureaucracy and into the Oval Office on time and then making certain the decisions are carried out. He has done this without impinging on President Reagan's desire to have his Cabinet officers--not an all-powerful White House staff adviser--as the main shapers of administration policy.

That is something Reagan wanted to do from the start but which did not work out with his first special assistant for national security affairs, Richard V. Allen.

Allen resigned under pressure in January after an ordeal following revelations that a $1,000 honorarium from a Japanese journalist intended for Mrs. Reagan had remained in a safe in Allen's office. Allen was cleared of any wrong-doing. It was clear, however, that the president wanted a stronger authority in NSC and Clark was transferred there, after serving a year as deputy to Secretary of State Alexander M. Haig Jr., with a greatly expanded mandate to whip the bureaucracy into shape.

Officials who have watched Clark and the president in action close up say Clark's new duties in the White House are grounded heavily in both men's belief that, despite the complex details, what is needed at the National Security Council level is "common sense."

Officials say that Reagan, with little experience in foreign affairs, often dealt with foreign leaders on the telephone early in his tenure with the help of 3-by-5 cards supplied by his staff. They say that these days a more confident president feels, as one put it, "that his advisers don't know much more than he does about these subjects."

"The Judge," as Clark is called, has made another crucial move that also contributes to the high marks thus far. He brought with him to the White House as his deputy a former Marine Corps colonel, Robert C. McFarlane, who worked on the NSC staff under Presidents Nixon and Ford, then on the staff of the Senate Armed Services Committee and then, last year, as State Department counselor under Haig.

McFarlane is described by one insider as "the substantive screen in front of the president."

"The bottom line about him," says a colleague on the NSC staff, "is that you can't snow him. If you are thinking of bamboozling Clark, forget it, because no paper is going to get to him without it going through McFarlane."

There are still big question marks, however.

The most immediate one is whether Clark and McFarlane can keep up what many officials say is a murderous pace and work-load caused, in part, by Clark's lack of background and by weaknesses in parts of the staff.

"McFarlane is indispensable," says a State Department official. "He's experienced and even-handed. But there is no doubt in my mind that he will be overcome unless he gets help."

Ultimately, the question is what advice Reagan will get in a crunch from the person closest to him. Will Clark become a much more powerful force than either he or the president now envisions and thus possibly get in over his head in a situation that requires long experience?

"He's not like McGeorge Bundy, Henry Kissinger, Brent Scowcroft or Zbigniew Brzezinski," an official said, referring to the well-known and experienced security advisers who served administrations from Kennedy through Carter. "He can't just close the door behind him and do the work himself in an emergency."

Those earlier advisers, however, served presidents who had intense interests in the details of foreign affairs, such as Nixon and Carter, or had big reputations and egos of their own and became powerful figures in their own right, such as Kissinger and Brzezinski.

"The paradox about Clark," says another official, is that "now we have an adviser who is fair, has great common sense, has a close relationship to the president, and is a perfect adjudicator" for settling internal squabbles and coordinating policy. "But being an adjudicator usually assumes expertise, as a judge who knows the law well. Clark knows the law, but everybody knows he doesn't know this well."

Nevertheless, Clark has established himself quickly, through his own efforts and manner and his close personal relationship with the president.

For the first time since 1969, when Kissinger transformed the adviser's job into a powerhouse, officials say major interagency papers and options meant for the president are going into the Oval Office without covering memos from the NSC adviser suggesting which option may be best. "This has produced greater openness," one official said. During the Nixon, Ford and Carter years, he "The paradox about Clark" is that "now we have an adviser who is fair, has great common sense, has a close relationshp to the president, and is a perfect adjudicator . . . . But being an adjudicator usually assumes expertise, as a judge who knows the law well. Clark knows the law, but everybody knows he doesn't know this well." said, "nobody really knew what final advice the president was getting."

White House officials say when Clark came in, it became apparent to him that the process of getting the president the best advice from his Cabinet members suffered from the lack of a "bad cop" to force solutions of disagreements that delayed and sometimes prevented presidential action.

Clark got Reagan personally to issue directives to Cabinet members to supply answers by specific dates to questions on a wide range of policy subjects. The questions are developed by NSC staff members. The answers are called national security study directives. After the president reads these and makes his choices, they are signed and become national security decision directives, which essentially are marching orders for the bureaucracy.

During all of 1981, officials say there were only about 14 of these decision directives. Some crucial questions, such as how American policy could be affected by the outcome of the Iran-Iraq war, were not addressed in these government-wide reviews, although it was studied by the State Department.

In almost five months under Clark, there have already been more than 20 such decision directives, officials said.

The first, and perhaps most crucial deals with an attempt to match overall American military strategy and national security objectives with the forces and costs needed to carry it out. The 3 1/2-month study is completed and approved and is expected to be made public in general outlines soon.

The work was carried out for Clark by Thomas C. Reed, a former secretary of the Air Force who Clark signed on as an NSC consultant. In one sense, the Reed operation suggests that the NSC staff is getting back to some degree into the business of formulating policy.

On the other hand, the fact that a consultant had to be brought in tends to demonstrate the relative weakness of the NSC staff and perhaps some mistrust of the Pentagon on this subject.

White House aides do not dispute the assessment that the Reagan-era NSC staff is, in relative terms, weaker than its predecessors. But they point out that under Kissinger, for example, the staff was meant to help form policy, and the adviser's office thus required an accumulation of leading experts. In contrast, Reagan started out intent on making the departments of government the developers of policy.

Nevertheless, interviews with officials in several agencies indicate a widespread view that Richard Allen, with some exceptions, put together too weak a staff both in terms of expertise and in knowing how to get things done in the bureaucracy.

One top-ranking official claims "Allen did a great disservice in staffing NSC with ideologues."

"Reagan is much more moderate when left alone," this official said, implying that Allen's staff choices narrowed the president's options by being reminders of previous campaign statements.

Allen flatly rejects such talk. "It is rubbish to say that hard-working people who believe in Ronald Reagan and supported the 1980 Republican platform are ideologues," he said.

Since Clark arrived, seven NSC staff members have left for a variety of reasons or been asked to leave.

Last week, Clark moved to plug some of the holes but in so doing reached generally into the same ideological pool as had Allen. Clark named George Washington University Prof. Gaston Sigur to serve as director of Asian affairs, and another professor, Lewis Tambs of Arizona State University, as a consultant on Latin American affairs.

Both men were considered earlier for other administration jobs, with the strong political support of Sen. Jesse Helms (R-N.C.). Helms unsuccessfully fought for Tambs to be ambassador to Panama, even though Tambs had opposed the Panama Canal treaties, and to be assistant secretary of state for Latin America. Helms had also fought for a spot in the State Department for Sigur.

In an effort to beef up defense expertise, which several sources claim has been sorely lacking, Clark has also brought in Air Force Gen. Richard Boverie to be director of defense programs. Boverie, who was a Pentagon deputy for international security policy, worked on the NSC staff during the Ford administration. He replaces Army Gen. Robert L. Schweitzer, who was fired by Allen eight months ago for giving an uncleared speech but whose job has remained vacant until now.