Deposit of soft sludge rising from the ground (4)

I believe the answer is:

dump

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'deposit' is the definition.
(I know that dump is a type of store)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for dump that I've seen before include "Offload" , "Get rid of" , "Unload" , "Ditch - rubbish tip" , "Repository for rubbish etc" .)

I've seen this clue in The Telegraph.

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