Shocking gash in skin of adult, horrified (6)
I believe the answer is:
aghast
'horrified' is the definition.
(I've seen this before)
'shocking gash in skin of adult' is the wordplay.
'shocking' indicates anagramming the letters (letters shocked or shaken).
'in' indicates putting letters inside.
'skin of' means to remove the middle letters.
'adult' with its centre removed is 'at'.
'gash' anagrammed gives 'ghas'.
'ghas' inserted into 'at' is 'AGHAST'.
(Other definitions for aghast that I've seen before include "Horror-struck" , "Confounded" , "Filled with terror" , "Horrified, filled iwth consternation" , "Struck with horror" .)