Unsettle sound stage (5)
I believe the answer is:
phase
'stage' is the definition.
(phasing is a kind of staging)
'unsettle sound' is the wordplay.
'unsettle' becomes 'faze' ('faze' can be a synonym of 'unsettle').
'sound' indicates a 'sounds like' (homophone) clue.
'faze' sounds like 'PHASE'.
(Other definitions for phase that I've seen before include "Shape into stage" , "Carry out in stages" , "Aspect of moon" , "Shape (anag.)" , "Any distinct time period" .)