Meat cut served up in excellent sauce (5)
I believe the answer is:
aioli
'sauce' is the definition.
(aioli sauce is a kind of sauce)
'meat cut served up in excellent' is the wordplay.
'meat cut' becomes 'loi' (I can't justify this - if you can you should believe this answer much more).
'served up' says the letters should be written backwards (in a down clue, going upwards).
'in' is an insertion indicator.
'excellent' becomes 'ai' (resembles 'Al', old-fashioned term for excellent).
'loi' in reverse letter order is 'iol'.
'iol' put within 'ai' is 'AIOLI'.
Can you help me to learn more?
(Other definitions for aioli that I've seen before include "Garlicky mayonnaise" , "Dressing" , "Sort of mayonnaise" , "garlic dip" , "Mayonnaise seasoned with mayonnaise" .)