Excellent old lady is not finished dressing (5)
I believe the answer is:
aioli
'dressing' is the definition.
(type of condiment or dressing)
'excellent old lady is not finished' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lady' becomes 'Liz' (short for Elizabeth).
'is not finished' means to remove the last letter.
'liz' with its last letter removed is 'li'.
'ai'+'o'+'li'='AIOLI'
(Other definitions for aioli that I've seen before include "A lamb joint; driven clouds" , "Mayonnaise seasoned with mayonnaise" , "Garlic mayo" , "garlic sauce" , "Garlic 9" .)