Break up a girl's group (7)
I believe the answer is:
disband
'break up' is the definition.
(I know that disband is a more specific form of break up)
'a girl's group' is the wordplay.
'a girl's' becomes 'Dis' (short for Diane or Diana).
'group' becomes 'band' (band can mean a group of people).
'dis'+'band'='DISBAND'
(Other definitions for disband that I've seen before include "Break up as a group" , "Cease to be a group" , "Scatter" , "Dissolve" , "shed" .)