Chart-topper set cut unmusical sound (5)
I believe the answer is:
noise
'unmusical sound' is the definition.
(noise can mean an unpleasant sound)
'chart-topper set cut' is the wordplay.
'chart-topper' becomes 'No I' (resembles No. 1, top of the chart).
'cut' means to remove the last letter.
'set' with its last letter taken off is 'se'.
'noi'+'se'='NOISE'
(Other definitions for noise that I've seen before include "Unstructured sound" , "Loud or disturbing sound" , "Rumour" , "Unwelcome sound" , "Unpleasant sounds" .)