Snake sound (4)
I believe the answer is:
hiss
'snake sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hiss that I've seen before include "(Make noise to) express disapproval" , "Sibilant sound made through the teeth" , "Snake's sibilant sound" , "Make disapproving sound" , "Audio interference" .)