Scottish girl’s deceit crushing simpleton (6)
I believe the answer is:
lassie
'scottish girl's' is the definition.
(I've seen this before)
'deceit crushing simpleton' is the wordplay.
'deceit' becomes 'lie' (I've seen this in other clues).
'crushing' means one lot of letters goes inside another.
'simpleton' becomes 'ass' (ass is a kind of simpleton).
'lie' placed around 'ass' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "''Will ye go ..., go'' (folksong)" , "Movie role for several dog stars" , "TV series collie dog" , "Scots girl - film dog" , "Film and TV dog" .)