Horrified at receiving a terrible gash (6)
I believe the answer is:
aghast
'horrified' is the definition.
(I've seen this before)
'at receiving a terrible gash' is the wordplay.
'receiving' indicates putting letters inside.
'a terrible' indicates anagramming the letters.
'gash' anagrammed gives 'ghas'.
'at' enclosing 'ghas' is 'AGHAST'.
(Other definitions for aghast that I've seen before include "Horror-stricken, filled with consternation" , "Filled with fear and consternation" , "Filled with horror or shock" , "Thunderstruck" , "Filled with consternation or horror" .)