Worker who's underemployed? (5)

I believe the answer is:

miner

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'worker' is the definition.
(miner is a kind of worker)

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

(Other definitions for miner that I've seen before include "Person who works underground" , "Underground worker" , "it's the pits, for me" , "One works underground" , "One tapping vein" .)

I've seen this clue in The Globe and Mail.

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