Every other bit extracted, darker gas remains (5)

I believe the answer is:

dregs

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'remains' is the definition.
(thesaurus)

I can't explain the remainder of the clue.

(Other definitions for dregs that I've seen before include "Sediment - rabble" , "Sediment at the bottom of a liquid" , "What's left" , "Bottom of the barrel" , "Most worthless parts" .)

I've seen this clue in The Guardian.

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