Is he fixing the hair of the dog? (6)
I believe the answer is:
setter
'the dog?' is the definition.
(setter is a type of dog)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for setter that I've seen before include "who is giving you problems right now?" , "Dog trained to scent game" , "stream of cattle" , "Hunting dog" , "Jelly, perhaps" .)