Stealthy assassin (5)
I believe the answer is:
ninja
'stealthy assassin' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ninja that I've seen before include "Ninjutsu expert" , "Teenage Mutant ... Turtle (toy)" , "Stealthy Japanese fighter" , "Stealthy Japanese spy" , "Teenage Mutant -- Turtles (TV/film)" .)