Machinery sound (4)
I believe the answer is:
whir
'machinery sound' 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 "Prolonged buzz" , "Spin with a humming sound" , "Make a humming noise" , "Rotate with hum" , "Soft swishing sound" .)