Appalled at gash that's festering (6)
I believe the answer is:
aghast
'appalled' is the definition.
(I know that appalled can be written as aghast)
'at gash that's festering' is the wordplay.
'that's festering' indicates anagramming the letters.
'at'+'gash'='atgash'
'atgash' is an anagram of 'AGHAST'.
(Other definitions for aghast that I've seen before include "Filled with fear and consternation" , "'Appalled, dismayed (6)'" , "Struck with horror" , "Amazed" , "Filled with consternation or horror" .)