Prisoner's trick (3)
I believe the answer is:
con
I believe this is a double definition.
'prisoner's' is the first definition.
(abbreviation for 'convict')
'trick' is the second definition.
(a con is a fraud or trick)
(Other definitions for con that I've seen before include "Informally swindle" , "Have a reason not to" , "one in pen?" , "Direct steering; study" , "This man is a swindler" .)