Extra named Dud cast (8)
I believe the answer is:
addendum
'extra' is the definition.
(I've seen this before)
'named dud cast' is the wordplay.
'cast' indicates anagramming the letters.
'named'+'dud'='nameddud'
'nameddud' anagrammed gives '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" , "Item appended to book" , "Supplement to a book" , "PS" .)