Horror-struck (6)
I believe the answer is:
aghast
'horror-struck' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aghast that I've seen before include "'Appalled, dismayed (6)'" , "Horrified, filled iwth consternation" , "What's carried in bag has teacher" , "Struck with horror" , "Stupefied with horror" .)