Overcome with horror (6)
I believe the answer is:
aghast
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">overcome with horror</span>' is the definition.<br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for aghast that I've seen before include "Filled with terror" , "struck with fear" , "Very unhappy" , "in awe" , "'Appalled, dismayed (6)'" .)