Old Japanese assassin (5)
I believe the answer is:
ninja
'old japanese assassin' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ninja that I've seen before include "Person skilled in Japanese martial art" , "agent from the east" , "Stealthy Japanese fighter" , "Japanese expert in stealth and martial arts" , "Japanese trained assassin" .)