Kind of acid in vinegar (6)
I believe the answer is:
acetic
'kind of acid in vinegar' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acetic that I've seen before include "Colourless pungent acid" , "Saucy'" , "in relation to acid" , "Like vinegar's acid" , "The acid in vingear" .)