Mother's pet dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(mastiff is a kind of dog)
'mother's pet' is the wordplay.
'mother's' becomes 'mas' (ma is a kind of mother).
'pet' becomes 'tiff' (I've seen this before).
'mas'+'tiff'='MASTIFF'
(Other definitions for mastiff that I've seen before include "one of his best friends?" , "Hound" , "Powerful, smooth-coated dog used for security work" , "Large, powerful, short-haired dog" , "Large short-haired dog breed" .)