One who refuses to snicker out loud at speaker (8)
I believe the answer is:
naysayer
'one who refuses' is the definition.
'to snicker out loud at speaker' is the wordplay.
'to snicker' becomes 'neigh'.
'out loud' shows a homophone (sound like).
'at' means one lot of letters go next to another.
'speaker' becomes 'sayer'.
'neigh' is a homophone of 'nay'.
'nay'+'sayer'='NAYSAYER'
(Other definitions for naysayer that I've seen before include "He refuses to agree" , "One who denies" , "Obstructionist" , "Yes, Aryan may be person who habitually denies or refuses" .)