Notice Edmund abused rider (8)
I believe the answer is:
addendum
'rider' is the definition.
(thesaurus)
'notice edmund abused' is the wordplay.
'notice' becomes 'ad' (short for advert).
'abused' indicates anagramming the letters.
'edmund' is an anagram of 'dendum'.
'ad'+'dendum'='ADDENDUM'
(Other definitions for addendum that I've seen before include "Thing to be added" , "Material added to the end of a book" , "Additional item at the end of a book" , "Supplementary text tacked on to the end" , "Piece added on" .)