Mum's pet is a dog (7)
I believe the answer is:
mastiff
'a dog' is the definition.
(mastiff is a kind of dog)
'mum's pet' is the wordplay.
'mum's' becomes 'mas' ('ma' can be a synonym of 'mum').
'pet' becomes 'tiff' (I've seen this before).
'mas'+'tiff'='MASTIFF'
'is' acts as a link.
(Other definitions for mastiff that I've seen before include "Large, powerful, short-haired dog" , "Large short-haired dog breed" , "Strong breed of dog" , "Hound" , "Powerful dog breed" .)