Mother's tough dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(mastiff is a kind of dog)
'mother's tough' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'tough' becomes 'stiff' (I've seen this before).
'ma'+'stiff'='MASTIFF'
(Other definitions for mastiff that I've seen before include "Strong breed of dog" , "Large short-haired dog breed" , "Large, powerful, short-haired dog" , "Powerful breed of watchdog" , "one of his best friends?" .)