Nocturnal sound (5)
I believe the answer is:
snore
'nocturnal sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for snore that I've seen before include "Make racous nocturnal sound" , "Make noises sleeping" , "Noise made while asleep" , "6 made by sleeper" , "How to keep someone awake while sleeping?" .)