Second river crossed in heading on — 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')
I don't understand how the remainder of the clue works.
(Other definitions for rubicon that I've seen before include "Cross it to pass the point of no return" , "Point of no return (which Caesar crossed in 49BC)" , "Stream in northern Italy" , "River crossed by Caesar in decisive move" , "Caeser crossed this river in a decisive step- no going back" .)