Swindle prisoner (3)
I believe the answer is:
con
'prisoner' is the definition.
(abbreviation for 'convict')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for con that I've seen before include "one in pen?" , "Peruse carefully or swindle" , "Study - swindle" , "Swindle or see" , "Swindle in draconian way" .)