Old dog star (6)
I believe the answer is:
lassie
'old dog star' 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 from Scotland and dog from film" , "TV series/film dog" , "girl found" , "''Will ye go ..., go'' (folksong)" , "Collie featuring in many films" .)