Serpent's sound (4)
I believe the answer is:
hiss
'serpent's sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hiss that I've seen before include "Sound of serpent" , "Urgent whisper" , "Wordless sound of disapproval" , "less welcome sign?" , "Make sibilant sound like snake" .)