Bitterly hostile (8)
I believe the answer is:
virulent
'bitterly hostile' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for virulent that I've seen before include "Highly infectious (disease)" , "bitterly hostile" , "Violent or poisonous from a liver nut" , "[OPPOSITE OF] Harmless" , "Highly poisonous" .)