Crossword Genius

Pure underwear, half with skirts off (6)

Ross

I believe the answer is:

vestal

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'pure' is the definition.
(I know that pure can be written as vestal)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for vestal that I've seen before include "Modest female" , "Like virgins of old" , "Woman dedicated to celibacy" , "Valets for old Roman virgin" , "Virgin charged with keeping alight the fire on the altar to a Roman goddess" .)

I've seen this clue in The Guardian.
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