Dog making mother rigid (7)
I believe the answer is:
mastiff
'dog' is the definition.
(breed of dog)
'mother rigid' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'rigid' becomes 'stiff' (synonyms).
'ma'+'stiff'='MASTIFF'
'making' acts as a link.
(Other definitions for mastiff that I've seen before include "Strong breed of dog" , "Powerful dog breed" , "Powerful, smooth-coated dog used for security work" , "Large short-haired dog breed" , "Big dog" .)