Dog improved with new start (second) (6)
I believe the answer is:
setter
'dog' is the definition.
(setter is a type of dog)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for setter that I've seen before include "Long-haired dog breed" , "Hunting dog" , "Dog found in street" , "puzzle constructor" , "Puzzle composer" .)