When in jail, inmates imitate a feudal mercenary (5)

I believe the answer is:

ninja

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

'mercenary' is the definition.
(I know that ninja is a type of mercenary)

'when in jail inmates imitate a feudal' is the wordplay.
'NINJA' is hidden within the letters.
But, I'm uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for ninja that I've seen before include "Animated turtle" , "Eastern agent" , "One who steals" , "Japanese spy or assassin" , "Fourteenth-century Japanese assassin" .)

I've seen this clue in the Sydney Morning Herald.

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