Polish painting of holy figure in ancient boundary stream (7)
I believe the answer is:
rubicon
'ancient boundary stream' is the definition.
'polish painting of holy figure' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'painting of holy figure' becomes 'icon'.
'rub'+'icon'='RUBICON'
'in' is the link.
(Other definitions for rubicon that I've seen before include "Caesar's fateful river" , "5 significantly crossed!" , "Stream in northern Italy" , "Point of no return (which Caesar crossed in 49BC)" , "Caesar's point of no return" .)