Fall out! Engage in battle! (5)
I believe the answer is:
fight
This clue is a double definition.
'fall out' is the first definition.
(fall out can mean to fight or argue)
'engage in battle' is the second definition.
(to fight someone is to battle them)
(Other definitions for fight that I've seen before include "Brawl; argue" , "Contest" , "Box" , "(Take part in) battle" , "Conflict" .)