Swear (to) (6)
I believe the answer is:
attest
'swear to' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for attest that I've seen before include "Confirm to be true" , "Corroborate" , "Bear formal witness to, give evidence" , "Declare to be true or genuine" , "Provide firm evidence" .)