Story about silly girl (6)
I believe the answer is:
lassie
'girl' is the definition.
(lassie is a kind of girl)
'story about silly' is the wordplay.
'story' becomes 'lie' (I've seen this before).
'about' is an insertion indicator.
'silly' becomes 'ass'.
'lie' going around 'ass' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "Screen canine" , "Canine film star" , "Collie featuring in many films" , "Cinema dog who went home" , "Hollywood film-star dog" .)