Once Spooner’s vilest passage (9)
I believe the answer is:
erstwhile
'once' is the definition.
(I know that once can be written as erstwhile)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for erstwhile that I've seen before include "In the past" , "Former, previous" , "Former - list where (anag)" , "Formerly" , "Quondam" .)