Okay ruse is ruined (4)
I believe the answer is:
sure
'okay' is the definition.
(I've seen this in another clue)
'ruse is ruined' is the wordplay.
'is ruined' indicates an anagram.
'ruse' is an anagram of 'SURE'.
(Other definitions for sure that I've seen before include "Completely confident that one is right" , "Positive" , "Definite" , "See 12" , "Safe" .)