Atrocious (7)
'atrocious' is the definition.
(synonyms)
This is all the clue.
(Other definitions for heinous that I've seen before include "Very wicked, maybe in-house" , "Dreadful" , "Iniquitous" , "Extremely evil" , "Shocking (offence)" .)
(Other definitions for hellish that I've seen before include "Infernally unpleasant?" , "Extremely unpleasant" , "Very difficult" , "Very much" , "Diabolical, infernal" .)