Appalled (6)
I believe the answer is:
aghast
'appalled' is the definition.
(I know that appalled can be written as aghast)
This is all the clue.
(Other definitions for aghast that I've seen before include "Horror-stricken, filled with consternation" , "struck with fear" , "Horrified, appalled" , "Deeply dismayed" , "Filled with terror" .)