Make certain; guarantee (6)
I believe the answer is:
ensure
I believe this clue is a double definition.
'make certain' is the first definition.
(synonyms)
'guarantee' is the second definition.
(I know that guarantee can be written as ensure)
(Other definitions for ensure that I've seen before include "See run (anag)" , "Guarantee" , "Make certain for the future" , "Make certain or safe" , "Make certain, secure" .)