Hum of pilotless aircraft (5)
I believe the answer is:
drone
I believe this is a double definition.
'hum' is the first definition.
(I have seen 'Continuous hum ' mean 'drone' so perhaps 'hum' could also mean 'drone')
'pilotless aircraft' is the second definition.
(synonyms)
'of' acts as a link.
(Other definitions for drone that I've seen before include "Bertie Wooster, say" , "Buzzing sound" , "Male honey bee, an idler" , "Remote controlled aircraft" , "Queen's mate" .)