Mr. Clark's ascendancy

American diplomacy once again seems to be under a cloud of uncertainty because of rivalries within the US foreign policy establishment. The matter Ss cause for concern. When George Shultz replaced Alexander Haig as secretary of state, it was hoped the change would bring new steadiness and calm - and consensus - to foreign policymaking. Mr. Shultz has amply demonstrated his reserves of patience and calm in the face of difficult problems. But he appears not to have the dominance of foreign policy which a secretary of state must have to be fully effective.

A spate of stories has surfaced in the news media all underscoring that it is now William Clark, President Reagan's national security adviser, who is taking control, often bypassing the secretary of state. The shift is giving Europeans pause as well. An editorial in the Economist, the respected British weekly, says that Mr. Clark ''is arrogating to himself power and influence over foreign policy at the expense of George Shultz.'' It observes that Mr. Clark, though politically close to the President, is a ''relative novice'' in the field of foreign affairs.

There is little quarrel that the US President needs a competent national security adviser, someone who can lay fefore him all the options on foreign policy and security issues. But should not responsibility for the formulation and implementation of policy reside in the secretary of state? And is the secretary's usefulness not diminished when it is perceived that someone else has greater influence? Interagency conflict seems to have become something of a national weakness in the past few years. One need only recall how Henry Kissinger eclipsed Secretary of State William Rogers and how Zbigniew Brzezinski's rivalry with Secretary of State Cyrus Vance finally led to the latter's resignation.

Now Mr. Shultz appears to be faced with the same phenomenon. He lost out in his battle to keep two high-ranking diplomats in the Central America field, and meantime Mr. Kissinger has seized the limelight as chairman of the new bipartisan commission on Central America. Mr. Shultz also seems to be playing virtually no role in the arms control talks (though he apparently is not keen to be involved here). An interagency commission on arms control is chaired by Mr. Clark. Also, current Middle East diplomacy is in the hands of a Clark aide who has no experience in the region.

All of this would perhaps be less noticeable if it were not for the fact that US foreign policy seems deadlocked. This is not to undervalue Mr. Shultz's accomplishments: steadying US relations with the European allies, strengthening ties with Japan, maintaining good liaison with Congress, and in general stabilizing US policy. But there have been no politically saleable ''big ticket'' items under his tenure. Stalemate deepens in the Middle East. An arms control agreement seems as far away as ever. The situation in Central America remains parlous.

It is speculated that, precisely because Mr. Reagan needs a foreign policy success in time for 1984, he has let his close aide and confidant assume more and more power. The theory is that Mr. Clark may produce quick results. Yet the fact remains that the national security adviser has had less than three years' experience in foreign and security affairs. He may be a fast learner. But his lack of knowledge and historical depth are worrisome - especially when combined with Mr. Reagan's own lack of experience in foreign policy and his tendency to view issues in largely ideological terms.

The President must be aware of the concern at home and abroad about Mr. Clark's growing role. He should also know that he cannot afford to lose his respected secretary of state - and that Mr. Shultz, though an accommodating team player, can be effectual only if he is seen as having the predominant voice in foreign policy. It is up to Mr. Reagan to assign him that voice. It is up to Mr. Shultz to make sure he has it.