Good girl in The Mirror ? (5)
I believe the answer is:
glass
'the mirror ?' is the definition.
(looking glass)
'good girl' is the wordplay.
'good' becomes 'g' (abbreviation).
'girl' becomes 'lass' (lass can informally mean a girl).
'g'+'lass'='GLASS'
'in' acts as a link.
(Other definitions for glass that I've seen before include "Goblet" , "Translucent, usually transparent, material" , "Transparent solid" , "Perhaps schooner" , "Philip --, US composer" .)