Break up a girl's gang (7)
I believe the answer is:
disband
'break up' is the definition.
(disbanding is a kind of breaking up)
'a girl's gang' is the wordplay.
'a girl's' becomes 'dis'.
'gang' becomes 'band' (I've seen this before**).
'dis'+'band'='DISBAND'
(Other definitions for disband that I've seen before include "Cease to be a group" , "Break up as a group" , "Dissolve" , "Scatter" , "shed" .)