To sum up, pet, it's a rich wine (6)
I believe the answer is:
muscat
'a rich wine' is the definition.
'Muscat' can be an answer for 'wine' (muscat is a kind of wine). I'm not sure about the 'a rich' bit.
'to sum up pet' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'pet' becomes 'cat' (cats are often kept as pets).
'sum' back-to-front is 'mus'.
'mus'+'cat'='MUSCAT'
'it's' acts as a link.
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" .)