Hellish (8)
I believe the answer is:
infernal
'hellish' is the definition.
(I know that hellish can be written as infernal)
This is the entire clue.
(Other definitions for infernal that I've seen before include "Of hell, or tiresome like a nuisance" , "Extremely annoying" , "Relating to the underworld" , "Hellish - damned" , "To do with hell, or informally detestable" .)