Professional killer (6)
'professional killer' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for gunman that I've seen before include "As killer" , "Eg, sniper" , "Sean O'Casey play 'Shadow of a . . . . . .'" , "Armed criminal" , "Armed person" .)
(Other definitions for hitman that I've seen before include "offer?" , "Hired gun" , "Assassin" , "killer" .)