Optimistic for certain (8)
I believe the answer is:
positive
This clue is a double definition.
'optimistic' is the first definition.
('positive' is associated in meaning with 'optimistic')
'certain' is the second definition.
('positive' can be similar in meaning to 'certain')
'for' is the link.
(Other definitions for positive that I've seen before include "Definite, sure" , "Certain, confident in opinion" , "Optimistic; certain" , "'Definite, categorical (8)'" , "Constructive, optimistic" .)