One is presumably properly drilled (4)

I believe the answer is:

well

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'one' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for well that I've seen before include "Lift-shaft" , "Underground source" , "run" , "Bore; very much" , "Issue forth" .)

I've seen this clue in the Evening Standard.

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