Sweet sound of buzzing insect (6)
I believe the answer is:
humbug
'sweet' is the definition.
(boiled sweet)
'sound of buzzing insect' is the wordplay.
'sound of buzzing' becomes 'hum' (a humming sound).
'insect' becomes 'bug' (bug is a kind of insect).
'hum'+'bug'='HUMBUG'
(Other definitions for humbug that I've seen before include "Pretentious talk or boiled sweet" , "Hypocrisy - bah!" , "False or hypocritical talk" , "that a sucker may enjoy" , "Hypocrisy and cant - ask Scrooge" .)