Zzzzz (5)
I believe the answer is:
snore
'zzzzz' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for snore that I've seen before include "Noise from a sleeper" , "Noise indicating sleep" , "Breathe noisily during sleep" , "Make noises sleeping" , "6 made by sleeper" .)