One able to fill some of the time at work (7)

I believe the answer is:

dentist

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

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

Can you help me to learn more?

(Other definitions for dentist that I've seen before include "Stinted (anag.)" , "Tooth expert" , "Teeth treater" , "Tooth specialist" , "One who treats teeth" .)

I've seen this clue in The Telegraph.

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