Crossword Genius

Old Spooner’s most useless key? (9)

Ross

I believe the answer is:

erstwhile

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'old' is the definition.
(I know that old 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 - list where (anag)" , "Quondam" , "Former, previous" , "Formerly" .)

I've seen this clue in The Times.
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