Timmy's dog (6)
I believe the answer is:
'timmy's dog' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lassie that I've seen before include "girl found" , "Girl from Scotland and dog from film" , "''Will ye go ..., go'' (folksong)" , "Canine film star" , "Collie featuring in many films" .)