Man in formal evening wear (4)
I believe the answer is:
male
'man' is the definition.
(thesaurus)
'in formal evening wear' is the wordplay.
'wear' indicates the answer is hidden within the clue (I've seen 'wearing' mean this (the answer is covered by or 'wearing' the clue)).
'MALE' is hidden in the letters of 'in formal evening'.
(Other definitions for male that I've seen before include "A gender" , "Lame sort of fellow" , "One cannot have babies" , "As are drakes, bulls etc." , "man?" .)