Horror-struck (6)
I believe the answer is:
aghast
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">horror-struck</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)