Not your source of wealth! (4)

I believe the answer is:

mine

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'source of wealth' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for mine that I've seen before include "Coal supplier" , "Pit; belonging to me" , "Coalpit" , "Rich source" , "Tunnel for extraction of coal etc" .)

I've seen this clue in the Evening Standard.

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