Sour-tasting substance (4)
I believe the answer is:
acid
'sour-tasting substance' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acid that I've seen before include "Corrosive substance" , "Sour - LSD" , "Bitter or sour" , "Bitter; biting" , "Sour to the taste" .)