THE REAGAN ADMINISTRATION had already answered -- in fact -- the question of whether it would approve the agreement by which the hostages were retrieved from Iran. It would and it did. But its statement of formal approval, granted after a four-week review, is still important -- for laying out the grounds of its acceptance and for indicating the way it is working up a policy in its policy area of fighting international terrorism.

The statement, issued under State Department aegis, is at pains to avoid any suggestion that the United States has obligations to Iran by virtue of the negotiations conducted by Jimmy Carter. Approval is based, instead, on "the overall interests of the United States." These interests are defined as the rights of American claiments, terrorist policy, obligations to third parties like Algeria and the U.S. position in the Persian Gulf, "including Iran."

This is a fair place to come out. It would have been repugnant to most Americans, not to say politically objectionable to the administration, to accept any moral obligation to honor an agreement made with kidnappers. Nor would it make sense for the United States to accept a political obligation to a regime as hostile -- and unstable -- as the one in Tehran. Mr. Reagan has political considerations too for putting some distance between himself and his predecessor's handiwork.

Still, the interests citied in the State Department text are real interests, worthy of being pursued for American objectives even if the process Iran gains some benefit from American fidelity to them. The administration had been urged to invoke international law and denounce the hostage agreement as made under duress. Fortunately, it chose to finesse the question. The United States has a large interest in seeing that international agreements reached by negotiations are honored.

The new statement says that acceptance of the Iran agreement represents no precedent. What does it represent? The statement doesn't precisely say, and it's probably just as well. Rightly, this administration believes that showing a readiness to accommodate, rather than a readiness to strike back, can invite hostage-taking and other forms of terrorism. Hence it wants to advertise that it cannot be bound by negotiation. Also rightly, however, it understands that it may wish to leave an opening for negotiation in some situations. Hence it sees the use of having others believe that in those situations it can be bound.

The new statement concludes: "The present administration would not have negotiated with Iran for the release of the hostages. Future acts of state-sponsored terrorism against the U.S. will meet swift and sure punishment." This formulation is meant to contribute to the general aura of deterrence and political authority the administration is trying to generate. On that level it should be useful.

But care must be taken in defining "state-sponsored terrorism." It could conceivably cover the Chilean government's murder of Orlando Letelier in Washington better than it could the initial offense by Iranian terrorists whose "state sponsorship" remains in contention. What about terrorism committed by elements with some sort (what sort?) of Soviet sponsorship? Libyan? Iraqi? What about terrorism sponsored by the United States? The administration's dedication to fighting terrorism is commendable. It is now coming to the hard part.