Crossword Genius

Drone with loose filament (5)

Ross

I believe the answer is:

thrum

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'drone' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for thrum that I've seen before include "Repetitive sound" , "Fringe, tassel of unwoven threads" , "Play monotonously" , "Make a continuous humming sound" , "Strum, hum or drum!" .)

I've seen this clue in The Telegraph.
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