On a single occasion in former times (4)
I believe the answer is:
once
I believe this is a double definition.
'on a single occasion' is the first definition.
(I've seen this in another clue)
'in former times' is the second definition.
(I've seen this before)
(Other definitions for once that I've seen before include "not repeatedly" , "In the old days" , "At some former time" , "... upon a time for the storyteller" , "A single time" .)