Sound beginning when hummingbird is rapidly returning (5)
I believe the answer is:
whirr
'sound' is the definition.
(whirring is a kind of sounding)
'beginning when hummingbird is rapidly returning' is the wordplay.
'beginning' suggests taking the first letters.
The initial letters of 'when hummingbird is rapidly returning' is 'WHIRR'.
(Other definitions for whirr that I've seen before include "Sound of vibrations" , "Sound of vibration" , "Humming sound" , "Spin with hum" , "Spin noisily" .)