As far as one can sink? (5)

I believe the answer is:

depth

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'as far as one can sink?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

This is the entire clue.

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(Other definitions for depth that I've seen before include "Downward extent" , "Thoroughness" , "Insight" , "Measure" , "Distance downwards" .)

I've seen this clue in the Evening Standard.

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