Girls in specs? Not good! (6)
I believe the answer is:
lasses
'girls' is the definition.
(lass is a kind of girl)
'specs? not good' is the wordplay.
'specs?' becomes 'glasses' ('glasses' can be a synonym of 'specs').
'not' suggests deleting specific letters.
'good' becomes 'g' (abbreviation).
'glasses' with 'g' taken away is 'LASSES'.
'in' acts as a link.
(Other definitions for lasses that I've seen before include "Lads and - -" , "Young ladies" , "Northern girls?" .)