Declare not to be true (4)
I believe the answer is:
deny
'declare not to be true' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for deny that I've seen before include "Controvert" , "Forbid; not agree" , "Declare not to be true" , "Forbid; contradict" , "Contradict, refuse" .)