Girl's story about donkey (6)
I believe the answer is:
lassie
'girl's' is the definition.
(lassie is a kind of girl)
'story about donkey' is the wordplay.
'story' becomes 'lie' (I've seen this before).
'about' means one lot of letters goes inside another.
'donkey' becomes 'ass' (I've seen this before).
'lie' enclosing 'ass' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "TV series collie dog" , "TV series/film dog" , "Film and TV dog" , "Girl from Scotland and dog from film" , "Scottish girl - canine movie star" .)