Sibilant sound (4)
I believe the answer is:
hiss
'sibilant sound' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hiss that I've seen before include "Urgent whisper" , "Show disapproval" , "Make sibilant sound of disapproval as at villain" , "showing contempt?" , "(Make noise to) express disapproval" .)