Horrified hag sat shivering (6)
I believe the answer is:
aghast
'horrified' is the definition.
(I know that horrified can be written as aghast)
'hag sat shivering' is the wordplay.
'shivering' indicates anagramming the letters.
'hag'+'sat'='hagsat'
'hagsat' is an anagram of 'AGHAST'.
(Other definitions for aghast that I've seen before include "'Appalled, dismayed (6)'" , "Horrified, filled iwth consternation" , "Filled with fear and consternation" , "in awe" , "Stupefied with horrer" .)