Undercover assassin in Japan! (5)

I believe the answer is:

ninja

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

This clue might be a double definition.

'undercover assassin' is the first definition.
'ninja' can be an answer for 'assassin' (I've seen this before). I'm not certain of the 'undercover' bit.

'japan' is the second definition.
I know that 'japan' relates to this answer.

'in' acts as a link.

Can you help me to learn more?

(Other definitions for ninja that I've seen before include "Teenage Mutant -- Turtles (TV/film)" , "Assassins" , "Old Japanese hired assassin" , "Former mercenary" , "Assassin from Japan" .)

I've seen this clue in The Irish Times.

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