Crossword Genius

Mend a dud after ordering supplementary material (8)

Ross

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" .)

I've seen this clue in The Times.
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