Killer since caught in dope offence (8)
I believe the answer is:
assassin
'killer' is the definition.
(assassin is a kind of killer)
'since caught in dope offence' is the wordplay.
'since' becomes 'as' (I've seen this in other clues).
'caught in' is an insertion indicator.
'dope' becomes 'ass'.
'offence' becomes 'sin' (I've seen this before).
'ass'+'sin'='asssin'
'as' put within 'asssin' is 'ASSASSIN'.
(Other definitions for assassin that I've seen before include "Political murderer" , "One making hits" , "Hired killer" , "Political/religious killer" , "Bravo!" .)