One employed to unpick seams? (5)

I believe the answer is:

miner

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'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I cannot understand how the remainder of the clue works.

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(Other definitions for miner that I've seen before include "Underground worker" , "One who digs for ore" , "Man of 49" , "Excavator" , "Bevin boy" .)

I've seen this clue in The Telegraph, The Globe and Mail, The Irish Independent, The Guardian and The Irish News.

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