sound like a snake (4)
I believe the answer is:
hiss
'sound like a snake' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hiss that I've seen before include "Sound of serpent" , "Make sibilant sound of disapproval as at villain" , "Boo" , "Urgently whisper" , "Show disapproval" .)