Horrifying (9)
'horrifying' is the definition.
(I know that horrifying can be written as frightful)
This is all the clue.
(Other definitions for frightful that I've seen before include "horrible" , "ghastly" , "Terrible, awful" , "nasty" , "very bad" .)
(Other definitions for atrocious that I've seen before include "Horrifyingly wicked" , "Terrible" , "Shocking" , "Horrifying" , "Extremely bad" .)