Dressing uncovered maid, not slim (5)
I believe the answer is:
aioli
'dressing' is the definition.
(type of condiment or dressing)
'uncovered maid not slim' is the wordplay.
'uncovered' indicates the central letters.
'not' becomes 'o' (I've seen this in other clues).
'slim' becomes 'li' (I can't explain this - if you can you should give a lot more credence to this answer).
The central letters of 'maid' are 'ai'.
'ai'+'o'+'li'='AIOLI'
(Other definitions for aioli that I've seen before include "Sort of mayonnaise" , "Pungent sauce" , "Garlic 9" , "Garlicky mayonnaise" , "garlic dip" .)