Crossword Genius

The setter's rich source (4)

Ross

I believe the answer is:

mine

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

I cannot understand how the remainder of the clue works.

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(Other definitions for mine that I've seen before include "Store" , "Donnybrook" , "Dig for minerals like coal" , "Pit; source" , "Rich source" .)

I've seen this clue in The Times.
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