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' indicates putting letters inside.
'one soft' becomes 'p' (musical abbreviation for piano, 'soft').
'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" .)