Belligerent person (9)
'belligerent person' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for warmonger that I've seen before include "Aggressor" , "Hawk" , "Hitler, perhaps" , "Jingoistic individual" , "Jingoist" .)
(Other definitions for battler that I've seen before include "One struggling" , "Fighter" .)