Biting, mordant (7)
I believe the answer is:
acerbic
I believe this is a double definition.
'biting' is the first definition.
(I've seen this before)
'mordant' is the second definition.
(thesaurus)
(Other definitions for acerbic that I've seen before include "Tasting sour or caustic in speech" , "Acrimonious" , "Harsh and sharp in speech" , "Astringent in tone" , "Sharp, forthright" .)