FOR YEARS "Andreas" has been a code word, sometimes even a curse word, in the State Department and assorted chanceries of the West. The familiar reference is to Andreas Papandreou, perennial challenger of the conservative, pro- American Greek establishment. Whether by his Marxist ideology or the influence of leftists in his socialist movement, it is said, "Andreas" is committed to breaking the ties--the American alliance, NATO, the Common Market--that bind Greece to the West.

But it's "Andreas" no more. "Mr. Papandreou" is the new prime minister of Greece. His party, which had doubled its vote in 1977, doubled it again on Sunday, burying the incumbents. It is a sharp break with a long tradition, and the element of personal vindication cannot be denied. Some part of his triumph, moreover, surely arises from his playing on the distaste felt by many Greeks at the thought of being manipulated over the years by that familiar deus ex machina of Greek politics, the "American Embassy." Americans accustomed to taking Greece for granted have reason for concern.

But whether they have reason for alarm or panic, or whether they would do the Western cause any good by showing these feelings, is another matter. True, Mr. Papandreou has made part of a career out of criticizing what he, and not only he, sees as an overbearing American presence in Greece and an attitude of insensitivity toward Turkey's invasion of Cyprus, on the one hand, and the colonels' dictatorship (1967- 1974), on the other. His warnings about going it alone, however, are qualified in fact if not in word.

For general security and for support in its contest with Turkey, Greece has nowhere to go but to NATO. Mr. Papandreou has shown he can distinguish between bargaining harder over the terms of American use of Greek bases and cutting Greece off. He may bargain harder, too, over the terms of Greek membership in the Common Market. But he knows the way to the referendum that would be needed to quit the Market is guarded by the fiercely pro-Market conservative president, Constantine Karamanlis.

Mr. Papandreou, an economist, has been given a mandate for change, but it makes sense to respect the common Greek view that it is mostly a mandate for domestic change. Greece is a country with severe economic and social ills of a sort that conservative governments have been unable to treat effectively. To count Mr. Papandreou out of the West or to set tests of his loyalty could divert him from the domestic concerns on which he plainly means to focus first; and it could aggravate precisely the discomfort the most anxious Americans profess to fear.