Nasty gash in skin of aunt, stupefied with horror (6)
I believe the answer is:
aghast
'stupefied with horror' is the definition.
(I've seen this before)
'nasty gash in skin of aunt' is the wordplay.
'nasty' is an anagram indicator (a nasty or wrong spelling).
'in' means one lot of letters goes inside another.
'skin of' means to remove the middle letters.
'aunt' with its middle taken out is 'at'.
'gash' anagrammed gives 'ghas'.
'ghas' inserted within 'at' is 'AGHAST'.
(Other definitions for aghast that I've seen before include "Filled with consternation or horror" , "Horrified, appalled" , "Filled with horror or shock" , "Amazed" , "Struck with horror" .)