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 a hidden word (I've seen 'wearing' mean this (the answer is covered by or 'wearing' the clue)).
'MALE' can be found hidden inside '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?" .)