Left a gang leader a prisoner (3)
I believe the answer is:
lag
'a prisoner' is the definition.
(slang term)
'left a gang leader' is the wordplay.
'left' becomes 'l' (common abbreviation).
'leader' suggests taking the first letters.
The first letter of 'gang' is 'g'.
'l'+'a'+'g'='LAG'
(Other definitions for lag that I've seen before include "Hang back" , "Drag one's feet" , "Fall behind; insulate" , "Wrap water-pipes; fall behine" , "Dawdle" .)