A year ago, when William P. Clark agreed to become deputy secretary of state, he did so only out of a sense of reluctant loyalty to President Reagan. Clark said he believed he could serve the public interest best by keeping the California Supreme Court seat from which he hurled conservative, law-and-order dissents against the philosophy of the court's liberal majority.

He also said he had misgivings about his foreign policy experience, which he jokingly noted had been limited to "72 hours in Santiago." Then, as has been recalled repeatedly in the past few days, the joke went sour when his Senate confirmation hearing turned into a shambles of incoherent testimony underscoring that he knew almost nothing about international affairs.

Yet, Clark was dug in at the White House yesterday, methodically beginning the task of clearing up the confusion left by the biggest shakeup of the Reagan presidency, one that on Monday saw Clark replace Richard V. Allen as the president's national security affairs adviser with vastly expanded powers that could make him the most important figure in the administration's machinery for determining diplomatic, defense and intelligence policy.

It's a long way from his former preoccupation with the relatively parochial nuances of the California legal code to the top echelon of White House decision-making. But Clark's success in traveling that road in less than a year was only the latest demonstration of his ability to confound those who have made the mistake of writing him off as a nonentity.

As a young man he was unable to complete either college or law school because of poor academic performance. Yet, it was to Clark that Reagan, then governor of California, turned in 1967 when he needed someone able to sort out the chaos of the governor's office and turn it into a smoothly functioning organization.

Similarly, in his years as a judge, Clark's opinions, although highly conservative, frequently won grudging praise from his adversaries on intellectual grounds.

Now the question is whether he will be able to turn in the same sort of performance in his new job of trying to bring order and precision to a national security policy that has been beset by intramural squabbling, personality conflicts and charges that it lacks clarity and direction.

The question is especially interesting because, while the security adviser's post is being vested anew with the powers it had in past administrations, Clark comes from a very different mold than most of the men who held the job before him.

The best known previous occupants of the post--McGeorge Bundy, Walt W. Rostow, Henry A. Kissinger, Zbigniew Brzezinski--were academicians who sought to apply their theoretical views to the shaping of various grand designs for a U.S. course in world affairs.

By contrast, even Clark's strongest boosters readily admit that, despite 11 months of on-the-job training at the State Department, his education in the broad range of foreign policy issues lags far behind the point where he could aspire to a similar role.

Instead, what caused Reagan to turn to Clark were his abilities as an administrator and, more importantly, as a mediator and conciliator who in his months at State demonstrated an almost uncanny knack for resolving or at least papering over disputes between his volatile boss, Secretary of State Alexander M. Haig Jr., and other ranking administration officials.

Clark did that by winning Haig's trust and confidence, while retaining his credentials as a member in high standing of the tight circle of Californians--among them presidential counselor Edwin Meese III, White House deputy chief of staff Michael K. Deaver and Defense Secretary Caspar W. Weinberger--who have been Reagan's closest political intimates since they served in his gubernatorial administration.

At State, Clark was constantly on the telephone to his old cronies at the White House and Pentagon, running interference for Haig on policy and jurisdictional disputes and smoothing over the dust-ups that frequently erupted into headlines.

Now, the president obviously hopes that Clark will be able to transfer that ability to the White House and get all the disparate parts of the national security machinery working in harmony.

How Clark plans to do that is not clear. Perhaps the best clue as to how he is likely to proceed lies in his record at the State Department.

In addition to serving Haig as a go-between with the White House, he cemented his relationship with the secretary through a willingness to take on any tasks that needed top-level attention and to immerse himself in the kind of crash-course boning up necessary to give him a reasonable familiarity with the problem.

A lot of that involved the sort of routine, often tedious business--management, internal reorganizations, selection of ambassadors and other personnel, legal questions--that rarely attracts much public attention but is vital to the day-to-day operations of the department.

In the process, department officials say, Clark probably learned more about the inner workings of State and its relations with other agencies than he did about substantive policy issues, and they predict that this knowledge is likely to be of considerable value for the coordinating part of his new job.

On policy questions, Clark's record is less clear. Haig sent him on a few modest diplomatic missions, largely as a learning exercise. But Clark also put his imprint on some of the issues tackled by the administration in its first year, and demonstrated what one subordinate calls "an approach that is conservative in a pragmatic rather than an ideological sense."

He has been the main overseer of the administration's evolving economic development plan for the Caribbean basin, supervised the shifting of the U.S. position on law of the sea from identification with Third World aspirations toward greater sympathy with the interests of U.S. business firms, and rode herd on the controversial effort to move human rights policy away from the activism of the Carter administration toward more quiet diplomacy.

On the surface, that list of assignments might seem to suggest that Haig entrusted Clark only with issues of secondary importance. However, department officials agree that Clark's influence and authority in policy matters had been expanding steadily, and probably would have become very substantial had he remained at State.

In a recent interview with The Washington Post, Clark described the division of responsibilities between him and Haig in this way: "I have no priorities, and I try to be a utility infielder. More and more, my role inside the department is double-checking and backstopping things that we're falling behind on in all areas.

"My job is trying to avoid what Al calls the grave error of the Vietnam era, when the government got so focused on one issue that we lost sight of other things that were of concern to both our friends and foes."

His words could turn out to be a description of how he intends to approach the security adviser's job. What Reagan seems to want is not a conceptual formulator of policy in the Kissinger mold, but an administrator who can double-check and backstop different initiatives to keep them on track and a mediator who can iron out differences, soothe ruffled feelings and keep people pulling together as a team.

It's a function that Clark filled to Reagan's satisfaction in the 1960s in California and again at the State Department this past year.

Now the president obviously is banking on Clark's being able to do it again on a wider scale that will embrace the White House, State, the Pentagon, the CIA and all those other corners of the federal bureaucracy that collectively comprise America's national security machinery.