Assassin kept in isolation in jail (5)
I believe the answer is:
ninja
'assassin' is the definition.
(I've seen this before)
'in isolation in jail' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NINJA' is hidden within 'isolation in jail'.
'kept' acts as a link.
(Other definitions for ninja that I've seen before include "Stealthy Japanese fighter" , "Japanese stealth fighter" , "Japanese spy or assassin" , "martial art expert" , "Medieval Japanese trained in martial arts, assassination and espionage" .)