Prison, also its inmate (7)

I believe the answer is:

brigand

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'its inmate' is the definition.
I can't tell whether this definition defines the answer.

'prison also' is the wordplay.
'prison' becomes 'brig' (I've seen this before).
'also' becomes 'and' (both can mean in addition).
'brig'+'and'='BRIGAND'

Can you help me to learn more?

(Other definitions for brigand that I've seen before include "Country robber" , "thief" , "Outlaw" , "Criminal" , "Robber in a gang" .)

I've seen this clue in The Mirror.

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