On as single occasion (4)

I believe the answer is:

once

(Other definitions for once that I've seen before include "in bygone days" , "Formerly - not repeated" , "Formerly; as soon as" , "Formerly, at one time" , "A single time" .)

I've seen this clue in The Mirror.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.