Sour liquid (4)
'sour liquid' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vinegar that I've seen before include "Acetic acid" , "Ill-temper" , "Salad dressing" , "Liquid used as condiment or food preservative" , "Sour liquid used as condiment" .)
(Other definitions for acid that I've seen before include "It is neutralized by alkali" , "Chemical compound" , "Bitter or sour" , "Caustic liquid of many kinds" , "Cutting" .)