Sound of bee is sweet, but not bug (3)
I believe the answer is:
hum
'sound of bee' is the definition.
(noise made by a bee)
'sweet but not bug' is the wordplay.
'sweet' becomes 'humbug' (boiled sweet).
'but not' indicates named letters should be taken away.
'humbug' with 'bug' taken out is 'HUM'.
'is' acts as a link.
(Other definitions for hum that I've seen before include "Smell strongly" , "Sound (of business?)" , "Drone - stink" , "Sing with closed lips" , "Reek" .)