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