Very bitter (5)
I believe the answer is:
acrid
'very bitter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acrid that I've seen before include "Direly pungent" , "Having a bitter taste or small, like smoke" , "Strong and sharp-smelling" , "Sarcastic" , "Bitterly pungent" .)