A dog trained to fight (7)
I believe the answer is:
terrier
'a dog' is the definition.
(terrier is a kind of dog)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for terrier that I've seen before include "One who volunteers to fight" , "(Yorkshire?) dog" , "Dog - army volunteer - land register" , "Breed of dog - volunteer soldier - estate register" , "Breed of dog - Territorial Army volunteer" .)