Springfield was 80% clean (4)

I believe the answer is:

dust

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'clean' is the definition.
(dusting is a kind of cleaning)

I can't explain the remainder of the clue.

(Other definitions for dust that I've seen before include "Stud (anag)" , "Powdered eart" , "Microscopic particles in the air" , "Sprinkle -- powder" , "Do household cleaning" .)

I've seen this clue in The Guardian.

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