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