Biting, as wit (4)
I believe the answer is:
acid
'biting as wit' 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 "Sour, sharp-tasting" , "bad rain" , "Caustic liquid of many kinds" , "Acerbic" , "Biting - tart" .)