Veto (8)
'veto' is the definition.
(I know that veto can be written as prohibit)
This is the entire clue.
(Other definitions for prohibit that I've seen before include "Formally forbid" , "Stop" , "Ban; prevent" , "Forbid, ban" , "Bar" .)
(Other definitions for overrule that I've seen before include "Prevail over - disallow" , "Disallow from a superior position" , "Reject (decision) using superior authority" , "Reverse another's decision" , "Turn down (objection)" .)