Good girl seen in mirror (5)
I believe the answer is:
glass
'in mirror' is the definition.
(I know that glass is a has part or parts of mirror)
'good girl' is the wordplay.
'good' becomes 'g' (abbreviation).
'girl' becomes 'lass' (lass can informally mean a girl).
'g'+'lass'='GLASS'
'seen' acts as a link.
(Other definitions for glass that I've seen before include "Container for drinking from" , "barometer" , "Balloon, say" , "Transparent substance" , "Watch-face cover" .)