Fib about a fathead (female) (6)
I believe the answer is:
lassie
'female' is the definition.
(lassie is a kind of female)
'fib about a fathead' is the wordplay.
'fib' becomes 'lie' (I've seen this before).
'about' means one lot of letters goes inside another.
'a fathead' becomes 'ass' (I've seen this in another clue).
'lie' going around 'ass' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "''Will ye go ..., go'' (folksong)" , "TV series/film dog" , "girl found" , "Collie featuring in many films" , "TV/film dog" .)