One refuses any different speaker (8)
I believe the answer is:
naysayer
'one refuses' is the definition.
(someone who says no to things)
'any different speaker' is the wordplay.
'different' indicates an anagram (letters placed in a different order).
'speaker' becomes 'sayer' (I've seen this in another clue).
'any' anagrammed gives 'nay'.
'nay'+'sayer'='NAYSAYER'
(Other definitions for naysayer that I've seen before include "He refuses to agree" , "Yes, Aryan may be person who habitually denies or refuses" , "Obstructionist" , "One who denies" .)