Causing horror (8)
I believe the answer is:
gruesome
'causing horror' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gruesome that I've seen before include "Repugnant" , "Go, resume being revolting or gruesome" , "Grisly, horrible" , "'Grisly, macabre (8)'" , "Repulsive, horrid" .)