Sadly, one may be in its depths (7)

I believe the answer is:

despair

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'sadly 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 remainder of the clue works.

Can you help me to learn more?

(Other definitions for despair that I've seen before include "Misery" , "Complete absence of hope" , "A spider (anag.)" , "First appearance" , "Opposite of 10" .)

I've seen this clue in The Guardian.

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