Filled with horror (6)
I believe the answer is:
aghast
'filled with horror' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for aghast that I've seen before include "What's carried in bag has teacher" , "Horror-struck" , "Filled with horror or shock" , "Thunderstruck" , "Filled with fear and consternation" .)