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' (a firm is a company).
'with' says to put letters next to each other.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'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" .)