Timeless puzzle causes depression (4)

I believe the answer is:

sump

'depression' is the definition.

I cannot understand how the rest of the clue works.

(Other definitions for sump that I've seen before include "Cesspool" , "Engine's oil reservoir" , "Pit collecting water/oil" , "Reservoir for oil etc" , "Crankcase reservoir" .)

I've seen this clue in The Guardian.

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