Remaining laundries processed without using Nitrogen (8)

I believe the answer is:

residual

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'remaining' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for residual that I've seen before include "Pertaining to what is left over" , "'Remaining, left over (8)'" , "Remaining; repeat royalty" , "Left over as a remainder" , "Leftover - used rail (anag)" .)

I've seen this clue in The Hindu.

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