Polish picture showing Italian river (7)
I believe the answer is:
rubicon
'italian river' is the definition.
(I've seen this in another clue)
'polish picture' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'picture' becomes 'icon' (synonyms).
'rub'+'icon'='RUBICON'
'showing' is the link.
(Other definitions for rubicon that I've seen before include "Caesar crossed this river to take irrevocable step" , "Limit" , "signifies a famous boundary" , "River, its crossing decisive" , "Its crossing irrevocable" .)