Foe, some infantrymen, eventually retreating (5)
I believe the answer is:
enemy
'foe' is the definition.
(enemy is a kind of foe)
'some infantrymen eventually retreating' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'retreating' is a reversal indicator.
'ymene' can be found hidden inside 'infantrymen eventually'.
'ymene' written backwards gives 'ENEMY'.
(Other definitions for enemy that I've seen before include "is against us" , "Division" , "Hostile opponent" , "other side (like 16)" , "Armed adversary" .)