Mirror informant changes sides (5)
I believe the answer is:
glass
'mirror' is the definition.
(glass is a kind of mirror)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for glass that I've seen before include "(Material used in) mirror" , "Vitreous material" , "measure of wine" , "Tumbler, mirror" , "Goblet" .)