Antagonist hiding among twenty men evidently returning (5)
I believe the answer is:
enemy
'antagonist' is the definition.
(I know that enemy is a type of antagonist)
'hiding among twenty men evidently returning' is the wordplay.
'hiding' says the answer is hidden in the clue.
'returning' shows that the letters should be reversed in order.
'ymene' is hidden in the letters of 'among twenty men evidently'.
'ymene' written backwards gives 'ENEMY'.
(Other definitions for enemy that I've seen before include "Antagonist" , "Opposing force" , "Hostile person, foe" , "other side (like 16)" , "is against us" .)