Hostile force in Yemen prepared (5)
I believe the answer is:
enemy
'hostile force' is the definition.
(I've seen this in another clue)
'yemen prepared' is the wordplay.
'prepared' is an anagram indicator.
'yemen' anagrammed gives 'ENEMY'.
'in' acts as a link.
(Other definitions for enemy that I've seen before include "Rival" , "Hostile opponent" , "other side (like 16)" , "Armed adversary" , "Opposing force" .)