According to Spooner, diver could earn extra (9)

I believe the answer is:

moonlight

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'extra' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for moonlight that I've seen before include "Illumination from the earth's satellite" , "Do extra work" , "Steerer of boat" , "Burn the midnight oil" , "Natural (and romantic?) illumination" .)

I've seen this clue in The Independent.

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