The Reagan administration has reached a tentative agreement with Panamanian strongman Manuel Antonio Noriega under which he will resign as defense chief but remain on in Panama for the time being, a U.S. official said Friday.

The official, insisting on anonymity, said the settlement is "less than ideal" because the administration had initially insisted that Noriega leave Panama upon his resignation as part of a transition to democratic rule."We have to settle for three-fourths of a loaf," the official said.

The administration had been concerned that any solution that allowed Noriega to remain in Panama could enable him to retain effective control even if he surrendered his post as commander of the Defense Forces.

But the official said that Noriega, once he steps down, would be like a "cop out of uniform" and would cease to be the dominant force in Panama's political life.

The timing of Noriega's resignation and the question of whether he eventually will go into exile remain up in the air and are subject to future negotiations, the official said.

The Washington Post said in Friday's editions that Noriega is known to prefer the Dominican Republic if he is forced into exile.

On Thursday, White House spokesman Marlin Fitzwater hinted that the administration was backing away from its demand that Noriega leave Panama as part of a settlement.

The administration goal, he said, was for Noriega to "leave power."

Fitzwater had indicated earlier the arrangement did not represent a policy change, but President Reagan and others had said in previous statements no acceptable settlement could be reached until Noriega left Panama.

Fitzwater said: "Our policy is that Gen. Noriega must go, which means leave power. We've talked about that a number of times. We have said we prefer him to leave Panama, but the policy issue is to leave power."