A young lady in glasses (4)
I believe the answer is:
'a young lady' is the definition.
(lass is a kind of young lady)
'in glasses' is the wordplay.
'in' indicates a hidden word.
'LASS' is hidden in the letters of 'glasses'.
(Other definitions for lass that I've seen before include "Damsel" , "Scottish girl" , "Young woman (-. Eng.)" , "Young woman, girl" , "Miss" .)