Caustic, biting (7)
I believe the answer is:
mordant
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">caustic</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >'<span class="explanation-format__original">biting</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for mordant that I've seen before include "Biting, scathing" , "(Of wit) sharp, critical" , "Acrimonious" , "Bitingly sarcastic" , "Corrosive" .)