Positive ruse is used (4)
I believe the answer is:
sure
'positive' is the definition.
('sure' can be similar in meaning to 'positive')
'ruse is used' is the wordplay.
'is used' indicates anagramming the letters.
'ruse' anagrammed gives 'SURE'.
(Other definitions for sure that I've seen before include "See 12" , "Feeling no doubt" , "Definite" , "Agreed" , "stable" .)