One stabs Polish prisoner in 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')
'one stabs polish prisoner' is the wordplay.
'one' becomes 'i' (Roman numeral).
'stabs' means one lot of letters goes inside another (some letters stab themselves into others).
'polish' becomes 'rub' (polish meaning rub to make shiny).
'prisoner' becomes 'con' (abbreviation for 'convict').
'rub'+'con'='rubcon'
'i' put inside 'rubcon' is 'RUBICON'.
'in' is the link.
(Other definitions for rubicon that I've seen before include "Caesar's fateful river" , "Caesar crossed this river in an irreversible step" , "No, I curb the fateful river" , "Italian stream" , "Point of no return (which Caesar crossed in 49BC)" .)