Police not positive about possessing large dog (6)
I believe the answer is:
collie
'dog' is the definition.
(collie is a kind of dog)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for collie that I've seen before include "Breed of dog" , "See 2" , "Sheepdog wth ling nose and shaggy hair" , "Type of sheepdog" , "Shepherd's dog" .)