Unused grounds (5)

I believe the answer is:

dregs

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'grounds' is the definition.
(synonyms)

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

(Other definitions for dregs that I've seen before include "Parts of least value" , "Leftover drink, sediment" , "Settlings or deposits" , "Liquid remnants with sediment" , "Sediment - rabble" .)

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

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