College head girl in form (5)
I believe the answer is:
class
'form' is the definition.
(I know that form can be written as class)
'college head girl' is the wordplay.
'college head' becomes 'c' (I've seen this in other clues).
'girl' becomes 'lass' (lass can informally mean a girl).
'c'+'lass'='CLASS'
'in' acts as a link.
(Other definitions for class that I've seen before include "School group" , "period" , "Excellent" , "Category; lesson" , "Order - quality" .)