Caustic, biting (7)
I believe the answer is:
mordant
I believe this is a double definition.
'caustic' is the first definition.
(I've seen this in another clue)
'biting' is the second definition.
(I've seen this before)
(Other definitions for mordant that I've seen before include "Biting, scathing" , "(Of wit) sharp, critical" , "Acrimonious" , "Bitingly sarcastic" , "Corrosive" .)