Assassin detained in prison in Japan (5)

I believe the answer is:

ninja

I'm a little stuck... Click here to teach me more about this clue!

'assassin' is the definition.
(I've seen this before)

'in prison in japan' is the wordplay.
'in' indicates a hidden word.
'NINJA' is hidden within 'prison in japan'.

'detained' acts as a link.
This may not be correct. It may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for ninja that I've seen before include "Assassin from Japan" , "Japanese mercenary warrior" , "Ninjutsu expert" , "Japanese assassin/spy" , "Medieval Japanese trained in martial arts, assassination and espionage" .)

I've seen this clue in The Independent.

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