Girl in form blowing top (4)
I believe the answer is:
lass
'girl' is the definition.
(lass can informally mean a girl)
'form blowing top' is the wordplay.
'form' becomes 'class' (synonyms).
'blowing top' means to remove the first letter.
'class' with its initial letter removed is 'LASS'.
'in' acts as a link.
(Other definitions for lass that I've seen before include "Young woman (-. Eng.)" , "Young woman, girl" , "Familiar form of address for female" , "Sweetheart" , "Sweet resident of Richmond Hill?" .)