A singular garment for the idle (5)

I believe the answer is:

slack

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'the idle' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for slack that I've seen before include "coal dust" , "Limp - negligent" , "bits of coal" , "Not taut" , "Powdery coal" .)

I've seen this clue in the Evening Standard.

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