Severe virus almost goes active (8)
I believe the answer is:
grievous
'severe' is the definition.
(I know that severe can be written as grievous)
'virus almost goes active' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'active' indicates anagramming the letters (the letters are active/moving).
'goes' with its last letter removed is 'goe'.
'virus'+'goe'='virusgoe'
'virusgoe' with letters rearranged gives 'GRIEVOUS'.
(Other definitions for grievous that I've seen before include "distressing" , "The "G" of GBH" , "Deplorable" , "Causing distress" , "This sort of bodily harms is serious" .)