murky - I'm not suggesting anything should be added to make the clue sound, I was arguing against that idea.
My suggestion is to treat 'Once' as a conjunction rather than an adjective, I think the clue has a better surface this way - and if that means dropping the indicator, then it's a price worth paying (well, for me it is, although I accept that for you it is not).
I'd be happier with your suggestion that 'this' manages to specify which of the two definitions leads to the answer if it wasn't followed by 'is' before the second definition - double definition clues usually work in this way. Answer length is a subsidiary indication, it should not be definitive.
My earlier 'boundaries of fairness' remark wasn't questioning the clue's structure (and I certainly wasn't expecting it to be so controversial in certain quarters!) I was referring to the practice of using very obscure double definitions to arrive at simple answers - this clue is a variation on that idea. I dislike the practice, as do many solvers, but setters like to twist the knife from time to time. Having said that, I think Gos's clues were very good both before and after adjustment - no mean feat.
I've enjoyed discussing this with you murky, thank you, but I'm taking up too much space with my ramblings, so I'll leave it there.