A buzzer used to be quiet (4)
I believe the answer is:
wasp
'a buzzer' is the definition.
(something that buzzes)
'used to be quiet' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'was'+'p'='WASP'
(Other definitions for wasp that I've seen before include "Insect with a sting in its tail" , "Stingy type" , "Swap for a stinging insect" , "White Anglo-Saxon Protestant" , "Being in the American elite" .)