Point of no return (7)
I believe the answer is:
rubicon
'point of no return' is the definition.
(as in the phrase 'crossing the Rubicon')
This is all the clue.
(Other definitions for rubicon that I've seen before include "signifies a famous boundary" , "River crossed by Caesar - no turning back" , "Italy/Gaul boundary" , "River crossed by Caesar in decisive move" , "Line fatefully gone over" .)