Scottish girl's untrue story about beast (6)
I believe the answer is:
lassie
'scottish girl's' is the definition.
(I've seen this in another clue)
'untrue story about beast' is the wordplay.
'untrue story' becomes 'lie' (story can mean a fabrication. I am not sure about the 'untrue' bit.).
'about' is an insertion indicator.
'beast' becomes 'ass' (donkey).
'lie' enclosing 'ass' is 'LASSIE'.
Can you help me to learn more?
(Other definitions for lassie that I've seen before include "Canine film star" , "Film and TV dog" , "girl found" , "dog in pictures" , "Young Scottish female" .)