Line of no return (7)

I believe the answer is:

rubicon

(Other definitions for rubicon that I've seen before include "River crossed by Caesar - no turning back" , "Caeser crossed this river in a decisive step- no going back" , "What's important to cross" , "Caesar crossed this river in an irreversible step" , "term used in 12 [PIQUET]" .)

I've seen this clue in the Sydney Morning Herald.

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