Appendix (7)
'appendix' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for addendum that I've seen before include "Dud ad-men dealt with in end of book" , "Item appended to book" , "something tacked on" , "Extra item added at end of book" , "Extra piece of text" .)
(Other definitions for addenda that I've seen before include "Items added to a text" , "appendages" , "Postscripts, supplements" , "collection of extra material" , "Printed appendices" .)