Horrified sat, trembling, beside the awful hag (6)
I believe the answer is:
aghast
'horrified' is the definition.
(thesaurus)
'sat trembling beside the awful hag' is the wordplay.
'trembling' indicates anagramming the letters.
'beside' means one lot of letters go next to another (I've seen this in other clues).
'the awful' is an anagram indicator.
'sat' is an anagram of 'ast'.
'hag' with letters rearranged gives 'agh'.
'ast' after 'agh' is 'AGHAST'.
(Other definitions for aghast that I've seen before include "Horrified, filled iwth consternation" , "'Appalled, dismayed (6)'" , "Filled with consternation or horror" , "Appalled, shocked" , "Thunderstruck" .)