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