Hear unmistakable sound of aircraft (8)
I believe the answer is:
triplane
'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" .)