Break up gang fight behind clubs (7)
I believe the answer is:
crumble
'break up' is the definition.
(thesaurus)
'gang fight behind clubs' is the wordplay.
'gang fight' becomes 'rumble' (I've seen this before).
'behind' is a charade indicator (letters next to each other).
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'rumble' after 'c' is 'CRUMBLE'.
(Other definitions for crumble that I've seen before include "Break down, fall apart" , "Type of pudding - disintegrate" , "(To) fragment; pudding" , "Fall to bits; a pudding" , "Break into small fragments" .)