Ruin’s certain, no one denies (7)
I believe the answer is:
recants
'one denies' is the definition.
(to recant is to deny a previous statement)
I don't understand how the remainder of the clue works.
(Other definitions for recants that I've seen before include "Retracts" , "takes back what was said" , "Withdraws declaration" , "Disclaims, takes back" .)