Crossword Genius

Very enthusiastic when Spooner's humble as dessert (4,2,7)

Ross

I believe the answer is:

keen as mustard

I'm a little stuck... Click here to teach me more about this clue!

'very enthusiastic' is the definition.
(I've seen this before)

'when spooner's humble as dessert' is the wordplay.
I cannot quite understand how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'as' is within the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for keen as mustard that I've seen before include "Intensely enthusiastic" , "Extremely eager" , "Most zealous" , "Very interested to get involved" .)

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