Propeller noise (4)
I believe the answer is:
whir
'propeller noise' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for whir that I've seen before include "Soft swishing sound" , "Make a humming noise" , "Spin with a humming sound" , "Prolonged buzz" , "Rotate with hum" .)