Guarantee, promise (6)
I believe the answer is:
assure
I believe this is a double definition.
'guarantee' is the first definition.
(I know that guarantee is a kind of assure)
'promise' is the second definition.
(I know that promise can be written as assure)
(Other definitions for assure that I've seen before include "Convince, promise" , "Cover with a policy" , "Guarantee, promise" , "Make (something) certain to happen" , "Convince, guarantee" .)