Intense, vehement (6)
I believe the answer is:
fierce
I believe this clue is a double definition.
'intense' is the first definition.
(similar in meaning)
'vehement' is the second definition.
(I know that vehement can be written as fierce)
(Other definitions for fierce that I've seen before include "Violent in temper, not gentle" , "Ferocious, intense" , "Intensely violent, savage" , "Menacing" , "Wild and savage" .)