Has he volunteered to serve doggedly? (7)
I believe the answer is:
'has he volunteered to serve doggedly?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'has he' is the wordplay.
I cannot quite see how this works, but
'he' could be 'terri' and 'terri' is found within the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for terrier that I've seen before include "Breed of dog - volunteer soldier - estate register" , "Small, short-bodied dog" , "inventory of property" , "Persistent person" , "Breed of dog - Territorial Army volunteer" .)