Certain, confident (8)
I believe the answer is:
positive
I believe this clue is a double definition.
'certain' is the first definition.
(similar in meaning)
'confident' is the second definition.
(I know that confident can be written as positive)
(Other definitions for positive that I've seen before include "Constructive, optimistic" , "Confident, very sure" , "Definite, sure" , "Sure, convinced" , "Of a number, greater than zero - that's certain" .)