Polish hero crossed by rebellious old Roman (7)
I believe the answer is:
rubicon
'crossed by rebellious old roman' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'polish hero' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'hero' becomes 'icon'.
'rub'+'icon'='RUBICON'
Can you help me to learn more?
(Other definitions for rubicon that I've seen before include "Its crossing irrevocable" , "Point of no return (which Caesar crossed in 49BC)" , "Caesar's fateful river" , "Stream in northern Italy" , "Cross it to pass the point of no return" .)