Shocked at gash that looks nasty (6)
I believe the answer is:
aghast
'shocked' is the definition.
(I know that shocked can be written as aghast)
'at gash that looks nasty' is the wordplay.
'that looks nasty' indicates an anagram (a nasty or wrong spelling).
'at'+'gash'='atgash'
'atgash' anagrammed gives 'AGHAST'.
(Other definitions for aghast that I've seen before include "Stupefied with horror" , "Appalled, shocked" , "'Appalled, dismayed (6)'" , "Taken aback with horror" , "Struck with horror" .)