Maid of Perth’s a fool caught in a falsehood (6)
I believe the answer is:
lassie
'maid of perth's' is the definition.
(Scottish term for a girl, eg in Perth)
'a fool caught in a falsehood' is the wordplay.
'a fool' becomes 'ass' (ass is a kind of fool).
'caught in' is an insertion indicator.
'a falsehood' becomes 'lie' (lie is a kind of falsehood).
'ass' inserted within 'lie' is 'LASSIE'.
(Other definitions for lassie that I've seen before include "Scottish girl - canine movie star" , "Hollywood film-star dog" , "Young Scottish female" , "Collie featuring in many films" , "TV series/film dog" .)