One opposed to other manual workers? (5)

I believe the answer is:

thumb

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

I can't explain the rest of the clue.

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(Other definitions for thumb that I've seen before include "Hitch-hiker's tool" , "Attempt to obtain a lift" , "One opposed to other manual workers" , "First digit" , "Tom, for example" .)

I've seen this clue in The Telegraph.

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