The depth of depression (5)

I believe the answer is:

nadir

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'the depth' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for nadir that I've seen before include "Rock bottom" , "Given lowest place" , "Lowest point, or time of deepest depression." , "Drain into the lowest point (5)" , "most unsuccessful point" .)

I've seen this clue in The Guardian and The Globe and Mail.

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