Only contains one soft note, frankly (6)
I believe the answer is:
openly
'frankly' is the definition.
(thesaurus)
'only contains one soft note' is the wordplay.
'contains' means one lot of letters goes inside another.
'one soft' becomes 'p' (musical abbreviation for 'piano').
'note' becomes 'e' (musical note).
'p'+'e'='pe'
'only' enclosing 'pe' is 'OPENLY'.
(Other definitions for openly that I've seen before include "In candid, up-front manner" , "(Done) without concealment" , "Without concealment or prevarication" , "Candidly, without concealment." , "In frank and candid manner" .)