On a single occasion (4)

I believe the answer is:

once

'on a single occasion' is the definition.
(I've seen this before)

This is the entire clue.

(Other definitions for once that I've seen before include "In days gone by" , "Quick examination" , "Cone (anag.)" , "At some time past" , "One-time - one time" .)

I've seen this clue in The Times, The Independent, the Sydney Morning Herald, The Mirror, The Telegraph and The Guardian.

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