Girl seen in mirror, topless (4)
I believe the answer is:
lass
'girl' is the definition.
(lass is a kind of girl)
'mirror topless' is the wordplay.
'mirror' becomes 'glass' (glass is a kind of mirror).
'topless' means to remove the first letter.
'glass' with its first letter taken off is 'LASS'.
'seen in' acts as a link.
(Other definitions for lass that I've seen before include "Sweetheart" , "Sweet resident of Richmond Hill?" , "Damsel" , "Familiar form of address for female" , "Girl (of Richmond Hill?)" .)