Break up group after girl has son (7)
I believe the answer is:
disband
'break up' is the definition.
(disbanding is a kind of breaking up)
'group after girl has son' is the wordplay.
'group' becomes 'band' (band is a kind of group**).
'after' means one lot of letters go next to another.
'girl' becomes 'di'.
'has' means one lot of letters go next to another.
'son' becomes 's'.
'di'+'s'='dis'
'band' after 'dis' is 'DISBAND'.
(Other definitions for disband that I've seen before include "Cease to be a group" , "shed" , "Scatter" , "Break up as a group" , "Dissolve" .)