Sour condiment (7)
I believe the answer is:
vinegar
'sour condiment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vinegar that I've seen before include "Pickling medium" , "Liquid used as condiment or food preservative" , "Ill-temper" , "Pickling agent" , "Sour liquid used as condiment" .)