Fill with horror (5)
I believe the answer is:
appal
'fill with horror' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for appal that I've seen before include "exhibiting dismay" , "To shock, dismay" , "Cause to be unpleasantly surprised" , "Greatly dismay" , "Scandalise" .)