Sound coming from cottonmouth is sibilant (4)
I believe the answer is:
hiss
'sound' is the definition.
(I know that hiss is a type of noise)
'coming from cottonmouth is sibilant' is the wordplay.
'coming from' says the answer is hidden in the clue.
'HISS' is hidden within 'cottonmouth is sibilant'.
(Other definitions for hiss that I've seen before include "S-s-snake's s-s-sound?" , "Sibilant sound made through the teeth" , "Wordless sound of disapproval" , "Sound of serpent" , "Show disapproval" .)