Prisoner gets one penny (4)
I believe the answer is:
coin
'penny' is the definition.
'prisoner gets one' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'gets' indicates putting letters inside (gets can mean captures or absorbs).
'one' becomes 'i' (Roman numeral).
'con' enclosing 'i' is 'COIN'.
(Other definitions for coin that I've seen before include "Penny, say?" , "Mint" , "Invent (a new word)" , "Metal used as money" , "Metal money disc" .)