Girl's fib about backside (6)
I believe the answer is:
lassie
'girl's' is the definition.
(lassie is a kind of girl)
'fib about backside' is the wordplay.
'fib' becomes 'lie' (I've seen this before).
'about' is an insertion indicator.
'backside' becomes 'ass' ('ass' can be a synonym of 'backside').
'lie' placed around 'ass' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "dog in pictures" , "TV series/film dog" , "TV/film dog" , "Scottish girl - canine movie star" , "Screen mutt - Scottish girl" .)