Impertinence one might get from a waiter (5)
I believe the answer is:
sauce
This clue is a double definition.
'impertinence' is the first definition.
(I've seen this before)
'one might get from a waiter' is the second definition.
(I've seen this before)
(Other definitions for sauce that I've seen before include "Flavourful relish" , "Dressing served with food" , "Cheek (sl.); ketchup" , "Worcester, say" , "(Semi)liquid flavouring" .)