Crossword Genius

Runs out of crowd in skimpy undergarment (5)

Ross

I believe the answer is:

thong

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'skimpy undergarment' is the definition.
(thong is a skimpy type of underwear)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for thong that I've seen before include "Leather strip that forms the flexible part of a whip: excuse for underwear" , "Insubstantial underwear" , "Skimpy underwear" , "scanty garment" , "Narrow strip of hide used as lash" .)

I've seen this clue in The Telegraph.
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