Mother, inflexible, getting dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(breed of dog)
'mother inflexible getting' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'inflexible' becomes 'stiff' (similar in meaning).
'getting' says to put letters next to each other.
'ma'+'stiff'='MASTIFF'
(Other definitions for mastiff that I've seen before include "Hound" , "Powerful, smooth-coated dog used for security work" , "one of his best friends?" , "Big dog" , "Powerful dog breed" .)