A very quiet finish before nine’s coda (8)
I believe the answer is:
appendix
'coda' is the definition.
(thesaurus)
'a very quiet finish before nine's' is the wordplay.
'very quiet' becomes 'pp'.
'finish' becomes 'end' (synonyms).
'before' means one lot of letters go next to another.
'nine' becomes 'IX' (roman numeral).
'a'+'pp'+'end'+'ix'='APPENDIX'
(Other definitions for appendix that I've seen before include "Addition to a book" , "Extra bit" , "It comes last" , "part of book" , "Extra section" .)