Head knocking bishop out on one occasion (4)
I believe the answer is:
once
'on one occasion' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
(Other definitions for once that I've seen before include "One-time - one time" , "At some time past" , "On one occasion" , "On a single (or past) occasion" , "Quick examination" .)