The sad mongrel - wretched pet (6,7)
I believe the answer is:
golden hamster
'pet' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'the sad mongrel wretched' is the wordplay.
'wretched' indicates an anagram.
'the'+'sad'+'mongrel'='thesadmongrel'
'thesadmongrel' anagrammed gives 'GOLDEN HAMSTER'.
Can you help me to learn more?
(Another definition for golden hamster that I've seen is " Rodent kept as a pet".)