Edmund died with a burst appendix (8)
I believe the answer is:
addendum
'appendix' is the definition.
('addendum' can be a synonym of 'appendix')
'edmund died with a burst' is the wordplay.
'died' becomes 'd'.
'with' is a charade indicator (letters next to each other).
'burst' indicates an anagram.
'edmund'+'d'+'a'='edmundda'
'edmundda' anagrammed gives 'ADDENDUM'.
(Other definitions for addendum that I've seen before include "Additional material at the back of a book" , "Material added to the end of a book" , "Extra item added to book or text" , "Supplementary text tacked on to the end" , "Addition at end of book" .)