Rescue dog losing tail (8)
I believe the answer is:
retrieve
'rescue' is the definition.
(I've seen this before)
'dog losing tail' is the wordplay.
'dog' becomes 'retriever' (eg a Golden Retriever).
'losing tail' means to remove the last letter.
'retriever' with its final letter taken away is 'RETRIEVE'.
(Other definitions for retrieve that I've seen before include "Go for and bring back" , "Get or bring back something" , "Recover" , "Rescue" , "Regain or fetch" .)