Made a sibilant sound (6)
I believe the answer is:
hissed
'made a sibilant sound' is the definition.
('hiss' can be a synonym of 'make a sibilant sound')
This is all the clue.
(Other definitions for hissed that I've seen before include "Sibilated" , "Made sibilant sound like snake" , "Jeered (6)" , "Derided" , "Showed contempt" .)