Old lady having row about small dog (7)
I believe the answer is:
mastiff
'dog' is the definition.
(I know that Mastiff is a type of dog)
'old lady having row about small' is the wordplay.
'old lady' becomes 'ma' (both can mean mother).
'having' says to put letters next to each other.
'row' becomes 'tiff' (tiff is a kind of row).
'about' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'tiff' after 's' is 'stiff'.
'ma' next to 'stiff' is 'MASTIFF'.
(Other definitions for mastiff that I've seen before include "Large short-haired dog breed" , "Big dog" , "Powerful, smooth-coated dog used for security work" , "Powerful dog breed" , "one of his best friends?" .)