Positive ruse failed (4)
I believe the answer is:
sure
'positive' is the definition.
('sure' can be similar in meaning to 'positive')
'ruse failed' is the wordplay.
'failed' is an anagram indicator.
'ruse' with letters rearranged gives 'SURE'.
(Other definitions for sure that I've seen before include "stable" , "certainly in the USA" , "Certain to be a ruse" , "Safe" , "Positive" .)