The gold-digger's department of expertise? (6)

I believe the answer is:

mining

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'expertise?' 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 remainder of the clue.

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(Other definitions for mining that I've seen before include "Excavating (coal, ore)" , "Laying down explosives" , "non-renewable industry" , "Extracting ores or coal from the earth" , "Excavating (ores)" .)

I've seen this clue in the Evening Standard.

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