Polish leaders from imperial city of Naples crossed by old Roman emperor at the point of no return (7)
I believe the answer is:
Here is my best explanation:
'the point of no return' is the definition.
('Rubicon' can be a synonym of 'point of no return')
'polish leaders from imperial city of naples' is the wordplay.
'polish' becomes 'rub' (polish meaning rub to make shiny).
'leaders from' suggests taking the first letters.
The initial letters of 'imperial city of naples' is 'icon'.
'crossed by old roman emperor at' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for rubicon that I've seen before include "ancient border", "Limit", "term used in 12 [PIQUET]", "Cross it to pass the point of no return", "Point of no return (which Caesar crossed in 49BC)", "cause of conflict when crossed", "Caesar crossed this river to take irrevocable step", "Line fatefully gone over", "Caesar's fateful river" and "Caesar crossed this river, a decisive move".)