To occupy oneself, start to fight (6)
I believe the answer is:
engage
'to occupy' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for engage that I've seen before include "attack" , "Become bound; attract" , "Employ or pledge" , "Employ, hire" , "start fighting" .)