Nix in the Oval Office (4)
I believe the answer is:
veto
'nix in the oval office' 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 "Overrule, forbid" , "Embargo" , "Refuse to authorise" , "Vote that blocks a decision" , "UN rejection" .)