A quiet finish in part of book is an addition (9)
I believe the answer is:
appendage
'an addition' is the definition.
(thesaurus)
'a quiet finish in part of book' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'finish' becomes 'end' (synonyms).
'in' is an insertion indicator.
'part of book' becomes 'page' (a page in a book).
'end' put into 'page' is 'pendage'.
'a'+'p'+'pendage'='APPENDAGE'
'is' acts as a link.
(Other definitions for appendage that I've seen before include "Something attached to a larger thing" , "Addendum" , "An outgrowth" , "Thing attached to another" , "Attached item" .)