Coal miner's dog? (6)

I believe the answer is:
collie
This clue might be a double definition.
'coal miner's' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'dog?' is the second definition.
(collie is a kind of dog)
Can you help me to learn more?
(Other definitions for collie that I've seen before include "Sheepdog wth ling nose and shaggy hair" , "Popular kind of sheepdog" , "shepherd's gofer" , "Breed of sheepdog" , "Sheep-herding dog" .)
