Just say no
I think it could be either:
'just say no' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for deny that I've seen before include "Refuse to grant" , "Refute, contradict" , "Disclaim" , "Negate" , "Disown" .)
'just say no' 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 "Ban; refuse to allow" , "Forbid; prohibition" , "Reject - vote (anag)" , "Power to reject legislation" , "Refuse permission" .)