The heat of battle? (4)
I believe the answer is:
fire
'the heat' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for fire that I've seen before include "There's no smoke without it" , "Blaze - sack" , "Sack; discharge (a gun)" , "Let off" , "Weaponry" .)