Forms for a hundred girls (7)
I believe the answer is:
classes
'forms' is the definition.
('class' can be a synonym of 'form')
'hundred girls' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'girls' becomes 'lasses' (lass can mean a girl).
'c'+'lasses'='CLASSES'
'for a' is the link.
(Other definitions for classes that I've seen before include "Groups with the same social or economic status" , "See 1" , "Categories" .)