Swear, assert (4)
I believe the answer is:
aver
I believe this clue is a double definition.
'swear' is the first definition.
(synonyms)
'assert' is the second definition.
(synonyms)
(Other definitions for aver that I've seen before include "Maintain - assert - allege" , "State; say" , "make a statement" , "Say firmly, declare" , "'Affirm, assert (4)'" .)