Sound coming from cottonmouth is sibilant (4)
I believe the answer is:
hiss
'sound' is the definition.
(hiss is a kind of sound)
'from cottonmouth is sibilant' is the wordplay.
'from' indicates the answer is hidden within the clue.
'HISS' can be found hidden inside 'cottonmouth is sibilant'.
'coming' acts as a link.
(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" .)