Rival soldiers retreating in case of emergency (5)
I believe the answer is:
enemy
'rival' is the definition.
(enemy is a kind of rival)
'soldiers retreating in case of emergency' is the wordplay.
'soldiers' becomes 'men' (I've seen this before**).
'retreating' shows that the letters should be reversed in order.
'in' is an insertion indicator.
'case of' means to remove the middle letters (outsides of).
'emergency' with its middle taken out is 'ey'.
'men' backwards is 'nem'.
'nem' inserted within 'ey' is 'ENEMY'.
(Other definitions for enemy that I've seen before include "Division" , "one in opposition" , "is against us" , "Time" , "Foe, adversary" .)