A singular occasion (4)

I believe the answer is:

once

'a singular occasion' is the definition.
(happened only once)

This is all the clue.

(Other definitions for once that I've seen before include "On a single (or past) occasion" , "in bygone era" , "In days past" , "in bygone days" , "... upon a time for the storyteller" .)

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

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