Certain to be optimistic (8)
I believe the answer is:
positive
I believe this is a double definition.
'certain' is the first definition.
('positive' can be similar in meaning to 'certain')
'optimistic' is the second definition.
('positive' is associated in meaning with 'optimistic')
'to be' acts as a link.
(Other definitions for positive that I've seen before include "sort of image" , "Definite, confident" , "Sure, convinced" , "Very sure, certain" , "Optimistic; certain" .)