Certain, positive (4)
I believe the answer is:
sure
This clue is a double definition.
'certain' is the first definition.
(I know that certain can be written as sure)
'positive' is the second definition.
(similar in meaning)
(Other definitions for sure that I've seen before include "See 12" , "Completely certain" , "Positive" , "Safe" , "stable" .)