Sound proof that one is asleep (5)

I believe the answer is:

snore

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'sound' is the definition.
(snore is a kind of sound)

I don't understand how the rest of the clue works.

(Other definitions for snore that I've seen before include "How to keep someone awake while sleeping?" , "Noise made while asleep" , "Make raucous nasal sound during sleep" , "6 made by sleeper" , "Breathe noisily during sleep" .)

I've seen this clue in the Evening Standard.

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