Buzzer was quiet (4)
I believe the answer is:
wasp
'buzzer' is the definition.
(something that buzzes)
'was quiet' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'was'+'p'='WASP'
(Other definitions for wasp that I've seen before include "Influential American" , "Insect often irritating in summer" , "Social worker" , "Middle class American" , "Swap for a stinging insect" .)