Glass in a set? (7)

I believe the answer is:

crystal

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'glass' is the definition.
(I know that crystal is a type of natural glass)

I can't explain the rest of the clue.

(Other definitions for crystal that I've seen before include "Type of clear-cut, lucent glass" , "Clear mineral, eg, quartz" , "Clear material such as quartz" , "Material of fortune-teller's ball" , "Glass-like quartz" .)

I've seen this clue in The Irish Independent.

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