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