Crossword Genius

Firm with new prisoner (3)

Ross

I believe the answer is:

con

'prisoner' is the definition.
(abbreviation for 'convict')

'firm with new' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'with' means one lot of letters go next to another.
'new' becomes 'n'.
'co'+'n'='CON'

(Other definitions for con that I've seen before include "one in pen?" , "Opposed" , "Swindle; steer" , "Trick, deceive" , "Swindle or trick someone" .)

I've seen this clue in The Mirror.
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