Yemen upset foe (5)
I believe the answer is:
enemy
'foe' is the definition.
(I know that foe can be written as enemy)
'yemen upset' is the wordplay.
'upset' indicates anagramming the letters.
'yemen' anagrammed gives 'ENEMY'.
(Other definitions for enemy that I've seen before include "Yemen (anag.)" , "Foe, adversary" , "Armed adversary" , "Hostile opponent" , "one in opposition" .)