Grief caused by a mishap with his gun (7)
I believe the answer is:
anguish
'grief' is the definition.
(I've seen this before)
'a mishap with his gun' is the wordplay.
'mishap with' is an anagram indicator.
'his'+'gun'='hisgun'
'a' anagrammed with 'hisgun' makes 'ANGUISH'.
'caused by' acts as a link.
(Other definitions for anguish that I've seen before include "Great suffering" , "Great (mental) pain" , "Angst" , "Grief" , "Heartache, great distress" .)