Excellent new form of oil, a dressing (5)
I believe the answer is:
aioli
'a dressing' is the definition.
(type of condiment or dressing)
'excellent new form of oil' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'new form of' indicates anagramming the letters.
'oil' anagrammed gives 'oli'.
'ai'+'oli'='AIOLI'
(Other definitions for aioli that I've seen before include "Garlic 9" , "Garlic mayo" , "Mayo, in part?" , "Garlicky mayonnaise" , "Mayonnaise seasoned with mayonnaise" .)