Vote, possibly, or the opposite (4)
I believe the answer is:
veto
'the opposite' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'vote possibly' is the wordplay.
'possibly' is an anagram indicator.
'vote' is an anagram of 'VETO'.
'or' is the link.
Can you help me to learn more?
(Other definitions for veto that I've seen before include "Vote to block a decision" , "Blocking vote" , "Right to reject a decision" , "A confused vote, altogether negative" , "UN rejection" .)