Clever us, never set up to reveal guarantee (6)
I believe the answer is:
ensure
'guarantee' is the definition.
(I know that guarantee can be written as ensure)
'clever us never set up to reveal' is the wordplay.
'to reveal' indicates a hidden word.
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'erusne' can be found hidden inside 'clever us never'.
'erusne' back-to-front is 'ENSURE'.
(Other definitions for ensure that I've seen before include "Make certain for the future" , "See run (anag)" , "Make certain, clinch" , "Guarantee" , "Make certain or safe" .)