THE REAGAN ADMINISTRATION is gamely trying to compose a single coherent policy toward the motley collection of insular and rim-land states composing the Caribbean basin. Probably it can't be done, if only because the collection is too motley -- ethnically, economically and politically. Still, it's an effort well worth undertaking. There is a region there its proximity does warrant a special American interest, and the very attempt to formulate a policy can be a political as well as an educative process.

The impulse to form a Caribbean basin policy recalls the fellow who asked his tailor to sew a shirt on this button. The button is El Salvador, which administration early on chose to make the demonstration project of its global anti-Soviet policy. The idea now seems to be at once to show that the United States is interested in fighting communism by more than military means and to enlist other states in a common definition or acceptance of a Cuba-centered security peril. Steps in economic development are being discussed and, at a faster pace, regional security plans.

Even without a comprehensive policy, this administration had moved to define its Caribbean relationships by a standard of pro-free enterprise and anti-communism. On this basis, the El Salvador junta has won broad support; aid to the Sandinista government in Nicaragua has been cut off; Jamaica, with a government friendly to American interests, has received special economic favor; and tiny Grenada, suspected of tilting toward Cuba, has been squeezed. The new policy presumably will refine and extend the premises expressed by these steps.

Steps taken bilaterally, however, are not likely to develop a sense of willing common enterprise with the United States. For that a broader process of common consultation and decision-making is required. If such a process is to work, however, it will not be on the basis of standards set only in Washington. The different Spanish- and English-speaking states will want to pose their own standards for a common regional policy. To the administration, the Cuban-Soviet security threat represents the preeminent fact of regional life. But few Caribbean nations will embrace that definition, or will embrace it with the administration's degree of commitment. To most of them, the cardinal requirement is for the United States to accept the diversity and legitimacy of local interests in their region.

At this point, the administration's own designs may be less important than the consultations it has been conducting with other countries, inside and outside the region. Consultation, if it is effective, may water down the administration's ideological prescriptions, but it may also help lay a more solid foundation on which to build consensus. That way good neighbor policies become real.