Spoil the sound of an instrument (4)
I believe the answer is:
loot
'spoil' is the definition.
(thesaurus)
'sound of an instrument' is the wordplay.
'sound of' indicates a 'sounds like' (homophone) clue.
'an instrument' becomes 'lute' (lute is a kind of instrument).
'lute' sounds like 'LOOT'.
'the' is the link.
(Other definitions for loot that I've seen before include "Burglar's takings - pillage" , "Steal goods as spoils" , "Stolen property" , "Stolen goods" , "familiar officer" .)