Sauce is top quality with oil mixed in (5)
I believe the answer is:
aioli
'sauce' is the definition.
(aioli is a kind of sauce)
'top quality with oil mixed in' is the wordplay.
'top quality with' becomes 'ai' (I can't explain this - if you can you should believe this answer much more).
'mixed' is an anagram indicator.
'in' indicates putting letters inside.
'oil' with letters rearranged gives 'iol'.
'ai' going around 'iol' is 'AIOLI'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for aioli that I've seen before include "Pungent sauce" , "Garlic-flavoured mayonnaise" , "A lamb joint; driven clouds" , "Garnish" , "Mayonnaise with garlic" .)