Certain to devise ruse (4)
I believe the answer is:
sure
'certain' is the definition.
(I know that certain can be written as sure)
'devise ruse' is the wordplay.
'devise' is an anagram indicator (I've seen 'devised' mean this).
'ruse' with letters rearranged gives 'SURE'.
'to' is the link.
(Other definitions for sure that I've seen before include "of course" , "Definite" , "Confident sort of ruse" , "The old sewer" , "Completely confident that one is right" .)