Sauce, still primarily considered the greatest, served up poorly (10)
I believe the answer is:
piccalilli
'sauce' is the definition.
The definition and answer can be both things that one consumes for nourishment as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'still primarily considered the greatest served up poorly' is the wordplay.
'still' becomes 'pic' (short for picture).
'primarily considered' becomes 'c' (I can't justify this - if you can you should believe this answer much more).
'the greatest' becomes 'Ali' (Muhammad Ali, known as the Greatest).
'served up' is a reversal indicator (in a down clue, letters go up).
'poorly' becomes 'ill' (similar in meaning).
'ill' reversed gives 'lli'.
'pic'+'c'+'ali'+'lli'='PICCALILLI'
Can you help me to learn more?
(Other definitions for piccalilli that I've seen before include "Mixed vegetables in a mustard sauce" , "Relish with chopped pickled cucumbers, green peppers and onion" , "Hot pickle" , "Chopped vegetables, mustard and hot spices" , "preserve with mustard" .)