Presidential thumbs down (4)
I believe the answer is:
veto
'presidential thumbs down' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for veto that I've seen before include "The power to prohibit or reject a proposal" , "Refuse to authorise" , "Don't allow" , "Vote that blocks a decision" , "Forbid outright" .)