Professional killer (6)

I think it could be either:

gunman

hitman

'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" .)

I've seen this clue in The Guardian.

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