Naysayer, say, exercising veto by right (5)
I believe the answer is:
voter
'naysayer say' is the definition.
(I've seen this in another clue)
'exercising veto by right' is the wordplay.
'exercising' is an anagram indicator (letters exercise or move around).
'by' is a charade indicator (letters next to each other).
'right' becomes 'r' (common abbreviation).
'veto' with letters rearranged gives 'vote'.
'vote'+'r'='VOTER'
(Other definitions for voter that I've seen before include "who may make one cross" , "One could use PR" , "One participating in ballot" , "supporter of politician" , "Overt (anag)" .)