Deserted a gang that's about finished (9)
I believe the answer is:
abandoned
'deserted' is the definition.
('abandon' can be a synonym of 'desert')
'a gang that's about finished' is the wordplay.
'gang' becomes 'band' (both can mean a group of people).
'that's about' is an insertion indicator.
'finished' becomes 'done' (both can mean completed).
'band' going around 'done' is 'bandoned'.
'a'+'bandoned'='ABANDONED'
(Other definitions for abandoned that I've seen before include "Given up?" , "Rejected" , "Crazy!" , "Left - dissolute" , "Deserted; uninhibited" .)