Hired assassin (6)
'hired assassin' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hit man that I've seen before include "He's a professional killer" , "Hired killer" , "Hired murderer" , "Hired gun" , "Liquidator" .)
(Other definitions for hitman that I've seen before include "Hired gun" , "offer?" , "Assassin" , "killer" .)