Firm with new prisoner (3)
I believe the answer is:
con
'prisoner' is the definition.
(abbreviation for 'convict')
'firm with new' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'with' means one lot of letters go next to another.
'new' becomes 'n' (abbreviation as in New Testament NT).
'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" .)