Girl fills reverse of dry forms (7)
I believe the answer is:
classes
'forms' is the definition.
('class' can be a synonym of 'form')
'girl fills reverse of dry' is the wordplay.
'girl' becomes 'lass' (lass is a kind of girl**).
'fills' is an insertion indicator.
'reverse of' shows that the letters should be reversed in order.
'dry' becomes 'sec' ('sec' can be similar in meaning to 'dry').
'sec' back-to-front is 'ces'.
'lass' put into 'ces' is 'CLASSES'.
(Other definitions for classes that I've seen before include "Groups with the same social or economic status" , "Categories" , "See 1" .)