Crossword Genius

Unrepeatable start of fairy tale (4)

Ross

I believe the answer is:

once

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'start of fairy tale' is the definition.
(once upon a time...)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for once that I've seen before include "Singly" , "On a single (or past) occasion" , "Previously" , "in days of yore?" , "On one occasion" .)

I've seen this clue in The Globe and Mail.
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