Gunman finding dope while on vice (8)
I believe the answer is:
assassin
'gunman' is the definition.
(thesaurus)
'dope while on vice' is the wordplay.
'dope' becomes 'ass'.
'while' becomes 'as'.
'on' means one lot of letters go next to another.
'vice' becomes 'sin'.
'ass'+'as'+'sin'='ASSASSIN'
'finding' is the link.
(Other definitions for assassin that I've seen before include "butcher" , "Asians SS? (Anag)" , "Hired murderer" , "One making hits" , "Liquidator" .)