Calm-sounding composition (5)
I believe the answer is:
piece
'composition' is the definition.
(synonyms)
'calm-sounding' is the wordplay.
'calm' becomes 'peace' (I've seen this before).
'sounding' indicates a 'sounds like' (homophone) clue.
'peace' is a homophone of 'PIECE'.
(Other definitions for piece that I've seen before include "king or queen?" , "Unpleasant character" , "A part of something" , "Single article" , "artefact" .)