Positive, certain (8)
I believe the answer is:
definite
This clue is a double definition.
'positive' is the first definition.
(I've seen this before)
'certain' is the second definition.
(similar in meaning)
(Other definitions for definite that I've seen before include "Unmistakable" , "The is the article" , "Explicit, positive" , "Certain; clearly stated" , "Clear-cut, certain" .)