Girl seen after midnight in the mirror (5)
I believe the answer is:
glass
'mirror' is the definition.
(looking glass)
'girl seen after midnight' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'seen after' means one lot of letters go next to another.
'midnight' becomes 'g' (the middle letter in 'night').
'lass' put after 'g' is 'GLASS'.
'in the' acts as a link.
(Other definitions for glass that I've seen before include "Goblet" , "Container for drinking from" , "Transparent stuff" , "Tumbler, mirror" , "Telescope perhaps" .)