Excellent oil used in mayonnaise (5)
I believe the answer is:
aioli
'mayonnaise' is the definition.
(mayonnaise seasoned with garlic)
'excellent oil used in' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'used' is an anagram indicator.
'in' indicates putting letters inside.
'oil' with letters rearranged gives 'iol'.
'ai' going around 'iol' is 'AIOLI'.
(Other definitions for aioli that I've seen before include "Garlic mayo" , "mayonniase" , "garlic dip" , "Mayonnaise with garlic" , "Pungent sauce" .)