To sum up, pet, it's a rich wine (6)
I believe the answer is:
muscat
'wine' is the definition.
(muscat is a kind of wine)
'to sum up pet it's a rich' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'sum' is 'mus' which is within the answer.
'pet' could be 'cat' (cats are often kept as pets) and 'cat' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for muscat that I've seen before include "Type of grape and the capital of Oman" , "Source of alcoholic drink" , "Sweet white wine - Omani port" , "Arabic port" , "Gulf port -- sweet grape" .)