A blocking vote (4)
I believe the answer is:
veto
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a blocking vote</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for veto that I've seen before include "Overrule, forbid" , "Refusal to assent" , "Reject - ban" , "Don't allow" , "Vote that blocks a decision" .)