Caustic — virulent (5)
I believe the answer is:
acrid
I believe this clue is a double definition.
'caustic' is the first definition.
(I know that caustic can be written as acrid)
'virulent' is the second definition.
(I know that virulent can be written as acrid)
(Other definitions for acrid that I've seen before include "Sharp - biting" , "causing corrosion" , "Bitterly pungent" , "Astringent" , "Unpleasantly bitter" .)