Composed part of a musical masterpiece (4)
I believe the answer is:
calm
'composed' is the definition.
(similar in meaning)
'part of a musical masterpiece' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'CALM' can be found hidden inside 'musical masterpiece'.
(Other definitions for calm that I've seen before include "Halcyon" , "Peaceful, undisturbed" , "Cool, composed" , "Tranquil, undisturbed" , "Cool and collected" .)