Crossword Genius

Hear unmistakable sound of aircraft (8)

Ross

I believe the answer is:

triplane

I'm a little stuck... Click here to teach me more about this clue!

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

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for triplane that I've seen before include "high flier once" , "old aircraft" , "Three-winged aircraft" , "Flyer" , "old flier" .)

I've seen this clue in the Sydney Morning Herald.
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