Sure is optimistic! (8)
I believe the answer is:
positive
I believe this clue is a double definition.
'sure' is the first definition.
('positive' can be similar in meaning to 'sure')
'optimistic' is the second definition.
('positive' is associated in meaning with 'optimistic')
'is' acts as a link.
(Other definitions for positive that I've seen before include "Certain, confident in opinion" , "Of a number, greater than zero - that's certain" , "sort of image" , "Optimistic; certain" , "Confident, very sure" .)