Maintain as true (6)
I believe the answer is:
affirm
'maintain as true' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for affirm that I've seen before include "State as a fact, perhaps in court" , "Make a formal declaration (rather than an oath)" , "Publicly state" , "Assert to be the case" , "Confidently put" .)