Doesn't breathe heavily enough for knickers (10)

I believe the answer is:

underpants

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But I can't explain it at all! Could you help me understand?

(Other definitions for underpants that I've seen before include "cover and support" , "Intimate clothing" , "Shorts" , "drawers" , "Close-fitting garment" .)

I've seen this clue in the Sydney Morning Herald.

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