Certain; optimistic (8)
I believe the answer is:
positive
I believe this is a double definition.
'certain' is the first definition.
(similar in meaning)
'optimistic' is the second definition.
(associated in meaning)
(Other definitions for positive that I've seen before include "'Definite, categorical (8)'" , "Sure, convinced" , "Optimistic; certain" , "Constructive, optimistic" , "Very sure, certain" .)