Post news, a conclusive piece (9)
I believe the answer is:
afterword
'a conclusive piece' is the definition.
(piece of writing at the end of a book)
'post news' is the wordplay.
'post' becomes 'after' (the prefix 'post-' means 'after').
'news' becomes 'word' (synonyms).
'after'+'word'='AFTERWORD'
(Other definitions for afterword that I've seen before include "I finish work" , "Final addition to book" , "book end" , "Concluding part of book" , "End matter" .)