Check depth of excellent bore-well in the beginning (5)

I believe the answer is:

plumb

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'well in the beginning' is the definition.
'plumb' can be an answer for 'well' (thesaurus). I am unsure of the 'in the beginning' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for plumb that I've seen before include "Precisely" , "Test alignment" , "Going straight up and down" , "Exactly - in a vertical line" , "Take soundings at sea" .)

I've seen this clue in The Hindu.

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