Assassin captured by policeman in Japan (5)
I believe the answer is:
ninja
'assassin' is the definition.
(I've seen this before)
'captured by policeman in japan' is the wordplay.
'captured by' indicates the answer is hidden within the clue.
'NINJA' is hidden in the letters of 'policeman in japan'.
(Other definitions for ninja that I've seen before include "Medieval Japanese trained in martial arts, assassination and espionage" , "Stealthy Japanese fighter" , "Assassin from Japan" , "Japanese spy or assassin" , "Japanese mercenary warrior" .)