A gold one, I'd be possessive about! (4)

I believe the answer is:

mine

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

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for mine that I've seen before include "Donnybrook" , "Pit; belonging to me" , "Excavate for minerals" , "Pit; source" , "Bomb - colliery" .)

I've seen this clue in the Evening Standard.

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