Young woman’s untrue statement about foolish person (6)
I believe the answer is:
lassie
'young woman's' is the definition.
(lassie is a kind of young woman)
'untrue statement about foolish person' is the wordplay.
'untrue statement' becomes 'lie'.
'about' indicates putting letters inside.
'foolish person' becomes 'ass' (ass can mean a foolish person).
'lie' placed around 'ass' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "Young Scottish female" , "TV series/film dog" , "dog in pictures" , "TV series collie dog" , "TV/film dog" .)