Polish artwork in stream Caesar crossed (7)
I believe the answer is:
rubicon
'stream caesar crossed' is the definition.
(Julius Caesar crossed the Rubicon)
'polish artwork' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'artwork' becomes 'icon' (icon is a type of artwork).
'rub'+'icon'='RUBICON'
'in' is the link.
(Other definitions for rubicon that I've seen before include "What's important to cross" , "Stream in northern Italy" , "term used in 12 [PIQUET]" , "Italian stream" , "River, point of no return" .)