Third of hornets extracted from swarm in underwear (5)

I believe the answer is:

thong

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'underwear' is the definition.
(I have seen 'Skimpy underwear' mean 'thong' so perhaps 'underwear' could also mean 'thong')

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for thong that I've seen before include "Strip of leather - minimalist beachwear" , "Strip of leather used for fastening" , "Lash of whip" , "Leather strip fastener" , "Narrow strip of hide used as lash" .)

I've seen this clue in The Guardian.

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