Yemen upset foe (5)
I believe the answer is:
enemy
'foe' is the definition.
(enemy is a kind of foe)
'yemen upset' is the wordplay.
'upset' is an anagram indicator.
'yemen' is an anagram of 'ENEMY'.
(Other definitions for enemy that I've seen before include "Yemen (anag.)" , "Foe, adversary" , "Armed adversary" , "Hostile opponent" , "one in opposition" .)