Pact is arranged, including suffering for drug dealer (10)
I believe the answer is:
pharmacist
'drug dealer' is the definition.
(I've seen this before)
'pact is arranged including suffering' is the wordplay.
'arranged' indicates anagramming the letters (letters rearranged).
'including' is an insertion indicator.
'suffering' becomes 'harm' (harm causes suffering).
'pact'+'is'='pactis'
'pactis' anagrammed gives 'pacist'.
'pacist' going around 'harm' is 'PHARMACIST'.
'for' is the link.
(Other definitions for pharmacist that I've seen before include "Drug dealer" , "Drugs dispenser" , "High-street chemist" , "Medicine dispenser" , "Drug dispenser" .)