Add a very quiet finish (6)
I believe the answer is:
append
'add' is the definition.
(I know that add can be written as append)
'a very quiet finish' is the wordplay.
'very quiet' becomes 'pp' (musical abbreviation for pianissimo).
'finish' becomes 'end' (synonyms).
'a'+'pp'+'end'='APPEND'
(Other definitions for append that I've seen before include "Add on at the bottom - say signature" , "Add (at the back of a document)" , "provided as add-on" , "Napped (anag)" , "Add (to end of text)" .)