Yemen upset rival (5)
I believe the answer is:
enemy
'rival' is the definition.
(enemy is a kind of rival)
'yemen upset' is the wordplay.
'upset' indicates an anagram.
'yemen' anagrammed gives 'ENEMY'.
(Other definitions for enemy that I've seen before include "is against us" , "Antagonist" , "Division" , "Armed adversary" , "Foe, adversary" .)