This produces profit in old gem reset (8)

I believe the answer is:

goldmine

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'this produces profit' is the definition.
The definition suggests a singular noun which matches the answer.

'in old gem reset' is the wordplay.
'reset' indicates an anagram.
'in'+'old'+'gem'='inoldgem'
'inoldgem' is an anagram of 'GOLDMINE'.

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I've seen this clue in The Sun.

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