Declare to be false (4)
I believe the answer is:
deny
'declare to be false' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for deny that I've seen before include "Forbid; not agree" , "Refuse; repudiate" , "Disown" , "Forbid; contradict" , "Refuse to accept" .)