Mother has stubborn dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(mastiff is a kind of dog)
'mother has stubborn' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'has' means one lot of letters go next to another.
'stubborn' becomes 'stiff' (I can't justify this - if you can you should give a lot more credence to this answer).
'ma'+'stiff'='MASTIFF'
Can you help me to learn more?
(Other definitions for mastiff that I've seen before include "Large short-haired dog breed" , "Powerful breed of watchdog" , "Large, powerful, short-haired dog" , "Hound" , "one of his best friends?" .)