A fetching thing for a sporting dog to do (8)
I believe the answer is:
'do' is the definition.
Although both the answer and definition are verbs in their base form, I cannot see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for retrieve that I've seen before include "Win back" , "Find and bring back" , "Search for and discover" , "Regain or fetch" , "Recover" .)