Hisses (4)
I believe the answer is:
boos
'hisses' is the definition.
('boo' can be a synonym of 'hiss')
This is all the clue.
(Other definitions for boos that I've seen before include "Shows disapproval" , "Shouts of disapproval" , "Catcalls" , "Shouts of derision" , "Sounds of disapproval" .)