Begin to fix a fight (3,5)
I believe the answer is:
set about
'begin' is the definition.
(I know that begin can be written as set about)
'fix a fight' is the wordplay.
'fix' becomes 'set' (synonyms).
'fight' becomes 'bout' (as in a bout in boxing).
'set'+'a'+'bout'='SET ABOUT'
'to' acts as a link.
(Other definitions for set about that I've seen before include "Begin - attack" , "Start to do something with determination" , "Embark upon" , "Get cracking" .)