Undertake to start a fight? (6)
I believe the answer is:
engage
'undertake' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for engage that I've seen before include "Mesh" , "Recruit" , "Take someone on as a worker" , "Hire (a worker)" , "Thief (rhyming slang)" .)