Excellent oil used for mayonnaise (5)
I believe the answer is:
aioli
'mayonnaise' is the definition.
(mayonnaise seasoned with garlic)
'excellent oil used' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'used' is an anagram indicator.
'oil' anagrammed gives 'oli'.
'ai'+'oli'='AIOLI'
'for' acts as a link.
(Other definitions for aioli that I've seen before include "Mayo, in part?" , "mayonniase" , "garlic sauce" , "Pungent sauce" , "Garlic mayo" .)