Annoying electronic sound endlessly made by buzzer (3)
I believe the answer is:
bee
'buzzer' is the definition.
(I've seen this before)
'annoying electronic sound endlessly' is the wordplay.
'annoying electronic sound' becomes 'beep'.
'endlessly' means to remove the last letter.
'beep' with its final letter taken away is 'BEE'.
'made by' is the link.
(Other definitions for bee that I've seen before include "queen, perhaps" , "Flying insect (often busy)" , "Honey generator" , "Stinging insect" , "competition" .)