Horrified at hag's writhing (6)
I believe the answer is:
aghast
'horrified' is the definition.
(thesaurus)
'at hag's writhing' is the wordplay.
'writhing' indicates anagramming the letters.
'at'+'hags'='athags'
'athags' with letters rearranged gives 'AGHAST'.
(Other definitions for aghast that I've seen before include "in awe" , "Appalled, shocked" , "'Horrified, shocked (6)'" , "Stupefied with horrer" , "Confounded" .)