Row involving old stray dog (7)
I believe the answer is:
terrier
'dog' is the definition.
(terrier is a kind of dog)
'row involving old stray' is the wordplay.
'row' becomes 'tier' (a tier is a row of things).
'involving' is an insertion indicator.
'old stray' becomes 'err' (to err is to stray or go wrong. I am not sure about the 'old' bit.).
'tier' going around 'err' is 'TERRIER'.
Can you help me to learn more?
(Other definitions for terrier that I've seen before include "Breed of dog - volunteer soldier - estate register" , "Breed of dog - Territorial Army volunteer" , "(Yorkshire?) dog" , "One who volunteers to fight" , "Part-time soldier" .)