THERE IS SOMETHING all backward about Richard Allen's decision to take a leave of absence from his duties as White House national security adviser to go to the public with his responses to the allegations that have been plaguing him for the past few weeks. No more than any other White House aide, does Mr. Allen have a "public" whom he must please in order to hold his post. He has a boss, one man, the president of the United States. Presumably Mr. Allen believes that his usefulness to his chief has not been impaired. But in the last analysis, this is for the president to decide.

Mr. Reagan evinces a strange diffidence, as Mr. Allen, reportedly on his own, takes to the ramparts full-time. Need he be reminded that he has a formidable investigative apparatus, called the Justice Department, already engaged in an examination of Mr. Allen's affairs, and that Congress has by law mandated the appointment of a special prosecutor, outside Justice, if that department cannot expeditiously lay to rest serious charges against a high federal official? Surely Mr. Reagan does not intend to let the Allen affair be resolved by a straw vote of the public--or, for that matter, by a straw vote of the White House inner circle.

At this point what is needed is not a defense of Mr. Allen by himself, but a specific hardheaded determination by the president of how the public trust is to be served in his administration and whether he thinks Mr. Allen has served it. It does no one any good to have Richard Allen twisting slowly, slowly, bereft of official support, while the president acts as though this were happening in an administration other than his own.