Monotonous sound (5)
(Other definitions for thrum that I've seen before include "Strum monotonously" , "Monotonously repeat" , "Tap a beat with fingers" , "Drum incessantly" , "Repetitive drone - end of weaver's thread" .)
'monotonous sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for drone that I've seen before include "Monotonous sound - member of hive" , "Talk boringly; a parasite" , "Low hum; parasite" , "Continuous sound; bee" , "Male bee for the queen" .)