Say quietly it’s Carol I’m upset by (8)
I believe the answer is:
proclaim
'say' is the definition.
(thesaurus)
'quietly it's carol i'm upset by' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'upset by' indicates an anagram.
'carol'+'im'='carolim'
'carolim' anagrammed gives 'roclaim'.
'p'+'roclaim'='PROCLAIM'
(Other definitions for proclaim that I've seen before include "Declare formally" , "Make a public announcement" , "Speak out" , "advertise" , "Broadcast" .)