Head bishop ousted? Not again! (4)
I believe the answer is:
once
'not again' is the definition.
(taking place only once - never again)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for once that I've seen before include "In the old days" , "In days past" , "One-time musical film and song" , "One-time - one time" , "In times past" .)