It's ref breaking up fighting (6)
I believe the answer is:
strife
'fighting' is the definition.
(I've seen this before)
'it's ref breaking up' is the wordplay.
'breaking up' indicates anagramming the letters.
'its'+'ref'='itsref'
'itsref' with letters rearranged gives 'STRIFE'.
(Other definitions for strife that I've seen before include "Conflict, quarrelling" , "Trouble and conflict" , "Contention or contest" , "Discord and friction" , "Bitter disagreement" .)