It's irrevocably crossed (7)

I believe the answer is:

rubicon

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But I can't explain it at all! Could you help me understand?

(Other definitions for rubicon that I've seen before include "ancient border" , "Caesar got over this" , "Cross it to pass the point of no return" , "Caeser crossed this river in a decisive step- no going back" , "Italian stream" .)

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