Row involving stray dog (7)
I believe the answer is:
Here is my best explanation:
'dog' is the definition.
(terrier is a kind of dog)
'row involving stray' is the subsidiary indication.
'row' becomes 'tier' (tier is a kind of row).
'involving' is an insertion indicator.
'stray' becomes 'err' ('err' can be a synonym of 'stray').
'tier' enclosing 'err' is 'terrier'.
(Other definitions for terrier that I've seen before include "Small, originally burrowing, dog", "Small, active breed of dog", "See 11", "Breed of dog - Territorial Army volunteer", "Reserve soldier", "Short-bodied canine", "Dog originally used to ferret out burrowing animals", "Small lively dog, may be Scotch", "Dog - reservist" and "Dog - army volunteer - land register".)