Postscript (8)
'postscript' is the definition.
(synonyms)
This is all the clue.
(Other definitions for addendum that I've seen before include "Extra piece of text" , "Addition at end of book" , "Extra item added to book or text" , "PS" , "Extra item added at end of book" .)
(Other definitions for epilogue that I've seen before include "Afterword" , "Closing section" , "End-of-play speech" , "Short postscript" , "Tailpiece - postscript" .)
'postscript' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for rider that I've seen before include "Person on horseback" , "Clause added to a legal document" , "it may be added" , "Possibly cavalier" , "Contract clause" .)