Declare; maintain (6)
I believe the answer is:
assert
This clue is a double definition.
'declare' is the first definition.
(I know that assert is a type of declare)
'maintain' is the second definition.
(I know that maintain can be written as assert)
(Other definitions for assert that I've seen before include "Swear" , "Maintain - allege" , "Declare, claim" , "State authoritatively" , "Aver, maintain" .)