Briefly toot buzzer (3)
I believe the answer is:
bee
'buzzer' is the definition.
(insect which buzzes)
'briefly toot' is the wordplay.
'briefly' means to remove the last letter.
'toot' becomes 'beep' (synonyms).
'beep' with its final letter taken away is 'BEE'.
(Other definitions for bee that I've seen before include "Social insect" , "Insect, may be in bonnet" , "Useful insect" , "Busy person" , "Honey generator" .)