Vinegary or vituperative (4)
I believe the answer is:
acid
I believe this is a double definition.
'vinegary' is the first definition.
(I've seen this in another clue)
'vituperative' is the second definition.
(thesaurus)
(Other definitions for acid that I've seen before include "Bitter substance" , "Corrosive fluid" , "Battery fluid" , "Sour tasting" , "Sour to the taste" .)