Female twit in fiction (6)
I believe the answer is:
lassie
'female' is the definition.
(I know that lassie is a type of fille)
'twit in fiction' is the wordplay.
'twit' becomes 'ass' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'fiction' becomes 'lie' (I've seen this before).
'ass' inserted into 'lie' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "Girl from Scotland and dog from film" , "''Will ye go ..., go'' (folksong)" , "Movie role for several dog stars" , "girl found" , "Young Scottish female" .)