Japanese assassin held in Benin jail (5)
I believe the answer is:
ninja
'japanese assassin' is the definition.
(I've seen this before)
'held in benin jail' is the wordplay.
'held in' says the answer is hidden in the clue.
'NINJA' can be found hidden inside 'benin jail'.
(Other definitions for ninja that I've seen before include "Stealthy Japanese spy" , "Ninjutsu expert" , "Person skilled in Japanese martial art" , "Assassin from Japan" , "Medieval Japanese trained in martial arts, assassination and espionage" .)