Hostile force some cavalrymen eventually repelled (5)
I believe the answer is:
enemy
'hostile force' is the definition.
(I've seen this in another clue)
'some cavalrymen eventually repelled' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'repelled' is a reversal indicator.
'ymene' is hidden in the letters of 'cavalrymen eventually'.
'ymene' backwards is 'ENEMY'.
(Other definitions for enemy that I've seen before include "Armed adversary" , "Hostile person, foe" , "Rival" , "Hostile opponent" , "Antagonist" .)