Drug dealer going after prison clerk (9)
I believe the answer is:
penpusher
'clerk' is the definition.
(penpusher is a kind of clerk)
'drug dealer going after prison' is the wordplay.
'drug dealer' becomes 'pusher' (someone who sells or pushes drugs).
'going after' means one lot of letters go next to another.
'prison' becomes 'pen' (I've seen this before).
'pusher' after 'pen' is 'PENPUSHER'.
(Other definitions for penpusher that I've seen before include "Bored clerk" , "Hen supper for an old scribe or lowly clerk" , "Clerical worker with a tedious job" , "Clerical worker with a boring job" .)