One who attacks first (9)
I believe the answer is:
aggressor
'one who attacks first' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aggressor that I've seen before include "The one who "started it"" , "One attacking without provocation" , "pirate comes into view" , "One acting hostilely" , "The one who starts the fight" .)