The opposite of bustles - yet still worn (6)

I believe the answer is:

slacks

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'the opposite' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for slacks that I've seen before include "Pants for casual wear" , "Avoids work - trousers" , "bags" , "casual attire" , "Works lazily; trousers" .)

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

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