Frightening, disturbing (8)
I believe the answer is:
alarming
This clue is a double definition.
'frightening' is the first definition.
(similar in meaning)
'disturbing' is the second definition.
(I've seen this before)
(Other definitions for alarming that I've seen before include "Worrying or disturbing" , "Dangerous" , "Striking with fear" , "Fearsome" , "Frightening or warning of danger" .)