Vet, Leo, with derringer, shot dog (6,9)
I believe the answer is:
golden retriever
'dog' is the definition.
(golden retriever is a kind of dog)
'vet leo with derringer shot' is the wordplay.
'with' is a charade indicator (letters next to each other).
'shot' is an anagram indicator.
'vet'+'leo'='vetleo'
'vetleo' put after 'derringer' is 'derringervetleo'.
'derringervetleo' anagrammed gives 'GOLDEN RETRIEVER'.
(Other definitions for golden retriever that I've seen before include "Dog" , "pet?" , "canine" .)