Mirror informant changes sides (5)

I believe the answer is:

glass

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'mirror' is the definition.
(glass is a kind of mirror)

I cannot understand how the rest of the clue works.

(Other definitions for glass that I've seen before include "(Material used in) mirror" , "Vitreous material" , "measure of wine" , "Tumbler, mirror" , "Goblet" .)

I've seen this clue in The Telegraph.

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