Once Spooner’s vilest passage (9)

I believe the answer is:

erstwhile

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'once' is the definition.
(I know that once can be written as erstwhile)

I don't understand the remainder of the clue.

(Other definitions for erstwhile that I've seen before include "In the past" , "Former, previous" , "Former - list where (anag)" , "Formerly" , "Quondam" .)

I've seen this clue in the Sydney Morning Herald.

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