Mend a dud after ordering supplementary material (8)
I believe the answer is:
addendum
'supplementary material' is the definition.
(something added)
'mend a dud after ordering' is the wordplay.
'after ordering' indicates anagramming the letters.
'mend'+'a'+'dud'='mendadud'
'mendadud' is an anagram of 'ADDENDUM'.
(Other definitions for addendum that I've seen before include "Extra item added to book or text" , "Dud ad-men dealt with in end of book" , "Postscript" , "Extra item added at end of book" , "Item appended to book" .)