Scathingly sarcastic (7)
I believe the answer is:
caustic
'scathingly sarcastic' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for caustic that I've seen before include "Burning, corroding" , "Chemically burning" , "Severe" , "Burning - cutting" , "Biting (wit); burning" .)