Problem with one prisoner is point of no return (7)
I believe the answer is:
rubicon
'point of no return' is the definition.
(as in the phrase 'crossing the Rubicon')
'problem with one prisoner' is the wordplay.
'problem' becomes 'rub' (rub can mean a problem or snag).
'with' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'prisoner' becomes 'con' (abbreviation for 'convict').
'rub'+'i'+'con'='RUBICON'
'is' is the link.
(Other definitions for rubicon that I've seen before include "Line fatefully gone over" , "Caeser crossed this river in a decisive step- no going back" , "Stream in north Italy that Julius Caesar crossed in 49 BC" , "Cross it to pass the point of no return" , "No, I curb the fateful river" .)