The Rubicon (5,2,2,6)
I believe the answer is:
point of no return
'the rubicon' is the definition.
('point of no return' can be a synonym of 'rubicon')
This is all the clue.
(Other definitions for point of no return that I've seen before include "after which one's committed?" , "After which one can't go back" , "Crucial moment" , "ace played?" .)