Sheep dog almost as big as a miner (6)
I believe the answer is:
collie
'sheep dog' is the definition.
(collie is a kind of sheep dog)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for collie that I've seen before include "Sheepdog with long pointed muzzle" , "Border dog" , "Breed of dog" , "Farm dog" , "Popular sheepdog like Lassie" .)