Girls with good spectacles (7)
I believe the answer is:
glasses
'spectacles' is the definition.
(I know this)
'girls with good' is the wordplay.
'girls' becomes 'lasses' (lass can informally mean a girl).
'with' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'lasses' after 'g' is 'GLASSES'.
(Other definitions for glasses that I've seen before include "Help in making out" , "Spectacles and receptacles" , "They make girls unattractive to Parker" , "Drinks" , "Spectacles; tumblers" .)