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