Two dopes and a smart killer (8)
I believe the answer is:
assassin
'killer' is the definition.
(assassin is a kind of killer)
'two dopes and a smart' is the wordplay.
'two dopes' means two substitutions for 'dope'.
'dope' becomes 'ass'.
'dope' becomes 'ass'.
'and' says to put letters next to each other.
'a smart' becomes 'in' (as in fashionable).
'ass'+'ass'+'in'='ASSASSIN'
(Other definitions for assassin that I've seen before include "Killer of prominent person" , "Hired killer" , "Hired murderer" , "Murderer, killer" , "Liquidator" .)