Chafe one prisoner to 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')
'chafe one prisoner' is the wordplay.
'chafe' becomes 'rub' (synonyms).
'one' becomes 'I' (Roman numeral).
'prisoner' becomes 'con' (abbreviation for 'convict').
'rub'+'i'+'con'='RUBICON'
'to' is the link.
(Other definitions for rubicon that I've seen before include "Stream in north Italy that Julius Caesar crossed in 49 BC" , "cause of conflict when crossed" , "River crossed by Caesar in decisive move" , "Polish picture depicting ancient" , "Stream in northern Italy" .)