Unrepeatable start of fairy tale (4)
I believe the answer is:
once
'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" .)