Who is that man at the President's elbow?

On a recent trip to the hinterlands one of this reporter's findings stood out in particular: no one seemed to be complaining about the people ''around the President.''

Four years ago it was difficult to talk to anyone without hearing criticism of those Jimmy Carter had brought in to work with him in Washington: Bert Lance, Hamilton Jordan, Peter Bourne, among others.

Also, one quickly finds that hardly anyone knows who President Reagan has at his elbow in the White House.

There has been some, but very little notice of Chief of Staff James Baker and Counselor to the President Edwin Meese.

Close presidential associate Michael Deaver somehow has kept out of sight.

But what seems most surprising is that the man who some knowledgeable observers say is fast becoming Mr. Reagan's right-hand man, National Security Adviser William P. Clark, is remaining pretty much an unofficial state secret, as far as the public is concerned.

When Mr. Clark replaced Richard Allen a few months ago, it was reported that the President was thinking of upgrading the role of his national security adviser to provide a foreign policy emphasis in the White House which he felt had been missing.

Clark, like Meese and Deaver, was someone that Mr. Reagan had known for years , going back to Sacramento days. On becoming governor, Reagan had made Clark his chief of staff. Later Reagan appointed Clark to the county bench and later to the California Court of Appeals and the California Supreme Court.

Before moving to the White House, Clark was serving as deputy secretary of state where, though relatively inexperienced in foreign affairs, he soon proved himself a quick learner and a particularly able coordinator.

So on Clark's recommendation, the President on Feb. 5 signed a directive calling for a study of military policy that would encompass all aspects of the administration's and the nation's response in the event of a confrontation.

This study, in the form of nine papers, now is complete. The first insight into the study came in Mr. Clark's recent speech at Georgetown University where he disclosed that the President has approved a new military strategy--one in which the United States forces need not engage those of the Soviet Union on all fronts simultaneously if a war breaks out.

Mr. Clark explains that taking into account its resources the US simply doesn't have the abilities to meet the Soviets on every front--and that, therefore, the US will have to look at priorities.

Mr. Clark further acknowledges that this change in policy is one that is based on the realities of the budgetary process.

The President now has signed a directive that details the implementation of the new strategy, a plan which those who have been involved in shaping it say could well be the blueprint on US abilities and priorities not only for this administration but for the balance of the century.

So it is clear that Mr. Clark is, indeed, playing a very active role over in the White House. The President has wanted to give more attention to foreign policy. And Clark is helping to bring this about.

Clark sees himself mainly as a coordinator, a broker of ideas, similar to his position as Governor Reagan's chief of staff. He gives no opinions himself, unless the President asks him specifically to say what he thinks about this or that.

Before Clark arrived in the White House, there had been conflicts between Allen and Haig and between Haig and Weinberger. White House insiders say that, through frequent communications with both Haig and Weinberger, Mr. Clark has kept such differences, which still sometimes occur, from becoming anything more than something the President can, and does, quickly resolve.

The casual onlooker at the presidency might well be of the opinion that Mr. Reagan still is spending most of his time on the budget. But some 35 percent of the President's scheduling time--more than for any other activity--now is spent on foreign policy.

Again, Mr. Clark is very much in the center of this new presidential preoccupation with foreign affairs, obviously the focal point of his second year in office. Yet, because of his quiet, understated manner--and an effort to keep a low profile--not many people, it seems, know who Clark is and what a key role he is playing.