Declare to be the case (4)
I believe the answer is:
aver
'declare to be the case' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aver that I've seen before include "'Affirm, assert (4)'" , "State firmly, declare" , "State (to be the case)" , "State; say" , "Maintain - assert - allege" .)