Dog is mother’s pet? (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'
'is' acts as a link.
(Other definitions for mastiff that I've seen before include "Powerful breed of watchdog" , "Strong breed of dog" , "Big dog" , "one of his best friends?" , "Powerful, smooth-coated dog used for security work" .)