Swindle a prisoner (3)
I believe the answer is:
con
I believe this clue is a double definition.
'swindle' is the first definition.
(I know that swindle can be written as con)
'a prisoner' is the second definition.
(abbreviation for 'convict')
(Other definitions for con that I've seen before include "Fleece" , "Defraud" , "one in pen?" , "Swindle, deception" , "Deception; disadvantage" .)