Lessons about girls (7)
I believe the answer is:
classes
'lessons' is the definition.
(as in school classes)
'about girls' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'girls' becomes 'lasses' (lass can mean a girl).
'c'+'lasses'='CLASSES'
(Other definitions for classes that I've seen before include "Categories" , "Groups with the same social or economic status" , "See 1" .)