First-class oil mixed with dressing (5)
I believe the answer is:
aioli
'dressing' is the definition.
(type of condiment or dressing)
'first-class oil mixed' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'mixed' is an anagram indicator.
'oil' anagrammed gives 'oli'.
'ai'+'oli'='AIOLI'
'with' is the link.
(Other definitions for aioli that I've seen before include "garlic dip" , "Sort of mayonnaise" , "Dressing" , "Mayonnaise seasoned with mayonnaise" , "Mayo, in part?" .)