Reservist having row about blunder (7)
I believe the answer is:
terrier
'reservist' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'row about blunder' is the wordplay.
'row' becomes 'tier' (tier is a kind of row).
'about' is an insertion indicator.
'blunder' becomes 'err' (I've seen this before).
'tier' going around 'err' is 'TERRIER'.
'having' acts as a link.
Can you help me to learn more?
(Other definitions for terrier that I've seen before include "Bedlington - -" , "inventory of property" , "part-timer in Services" , "Short-bodied canine" , "(Yorkshire?) dog" .)