Washington — President Reagan's expected appointment of William P. Clark to be his national security adviser would have seemed unthinkable a year ago.
Today the appointment of Deputy Secretary of State Clark, a man relatively inexperienced until recently in foreign affairs, appears from a Washington viewpoint to be the most logical of choices.
This is largely because Mr. Clark has proved himself at the State Department to be an effective mediator of bureaucratic disputes and coordinator of options and paper work -- a role similar in some ways to the one he would play as National Security Council (NSC) adviser. He also is a well-liked ''team player, '' in the words of some officials, and a close friend of Mr. Reagan's.
Richard V. Allen, the current national security adviser, took a leave of absence several weeks ago pending completion of investigations into his conduct while in that position. The Justice Department has cleared Mr. Allen of any criminal wrongdoing for accepting from a Japanese magazine a $1,000 honorarium -- which he left in his office safe -- and for accepting two gift watches from Japanese friends. But an internal White House review of Allen's conduct is continuing.
The controversy over Allen helped intensify a review of the White House foreign policy machinery -- a review apparently begun prior to Allen's leave. There have been complaints from within the bureaucracy that Allen was ineffective in coordinating foreign policy. More often than not, however, observers simply said that Allen and his NSC staff rarely counted for anything one way or the other in fashioning foreign policy. Allen's direct access to the President was reduced. White House counselor Edwin Meese III became the White House official, aside from the President, who dealt most authoritatively with foreign policy.
But perhaps in part because of inexperience in foreign affairs and because activities demanded so much of him, Mr. Meese did not appear to fill the lead role adequately. Vice-President George Bush, Reagan's crisis manager, took the lead in a number of policy-making meetings on Poland. But a feeling spread throughout the White House and the rest of the bureaucracy that the system must be revamped.
For one thing, a weak National Security Council staff and a lack of adequate bureaucratic coordination were allowing battles between the State and Defense Departments to erupt at every turn. It would not do for an administration that had come to power criticizing ''disarray'' in the Carter administration's foreign policy machinery to fall victim to the same difficulty.
As a result, Reagan is reported to be thinking of ''upgrading'' the role of his NSC adviser to provide a foreign policy focus in the White House that has been lacking. Allen came to office vowing never to repeat the performance of Henry A. Kissinger and Zbigniew Brzezinski in the NSC post. They were seen as having gone too far in the direction of making foreign policy, rather than helping to coordinate it, thus diminishing the authority of the secretary of state.
Clark has the advantage of being able to get along well with Secretary of State Alexander M. Haig Jr.
A certain amount of tension between the NSC adviser and secretary of state seems almost inevitable. Their roles force them to see things from different perspectives. But if Clark, a judge who was Reagan's chief of staff in California, gets the NSC job, he is not likely to pose the same kind of threat to Haig's authority that some foreign policy experts might pose.
Clark's rise in the esteem of foreign policy professionals at the State Department has been phenomenal. He started out at his confirmation hearings last year admitting that he knew little about foreign policy. But he has been a quick learner who just as quickly convinced the professionals, including Mr. Haig, that it was useful to have Clark around to smooth over differences between the State Department and Clark's friends in the White House.
Those in the State Department who suspected that Clark would be an ideological ''hatchet man'' for the White House were reassured when they saw his pragmatic side.