Prisoner has one dime, say (4)
I believe the answer is:
coin
'dime say' is the definition.
(dime is a kind of coin)
'prisoner has one' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'has' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'con' placed around 'i' is 'COIN'.
(Other definitions for coin that I've seen before include "Think up" , "Mint" , "Invent a word or phrase - for money?" , "Invent (a new word)" , "Metal used as money" .)