Horrified at hag's brew (6)
I believe the answer is:
aghast
'horrified' is the definition.
(thesaurus)
'at hag's brew' is the wordplay.
'brew' indicates an anagram.
'at'+'hags'='athags'
'athags' with letters rearranged gives 'AGHAST'.
(Other definitions for aghast that I've seen before include "struck with fear" , "Stupefied with horrer" , "Appalled, filled with consternation" , "Stupefied with horror" , "What's carried in bag has teacher" .)