Volunteer soldier’s mistake involved in row (7)
I believe the answer is:
terrier
'volunteer soldier's' is the definition.
(I've seen this before)
'mistake involved in row' is the wordplay.
'mistake' becomes 'err' (to err is to make a mistake or blunder).
'involved in' indicates putting letters inside.
'row' becomes 'tier' (a tier is a row of things).
'err' inserted within 'tier' is 'TERRIER'.
(Other definitions for terrier that I've seen before include "Bedlington - -" , "Dog - reservist" , "Persistent person" , "Dog originally used to ferret out burrowing animals" , "(Yorkshire?) dog" .)