Choose to veto, possibly (4)
I believe the answer is:
vote
'choose' is the definition.
(voting is a kind of choosing)
'veto possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'veto' is an anagram of 'VOTE'.
'to' acts as a link.
(Other definitions for vote that I've seen before include "Make cross" , ""X" marked on election day" , "Mark on ballot paper" , "Put one's cross" , "Mark choice in election" .)