Assume the setter’s optimistic? (8)
I believe the answer is:
positive
'optimistic?' is the definition.
('positive' can be similar in meaning to 'optimistic')
'assume the setter's' is the wordplay.
'assume' becomes 'posit' (I've seen this in another clue).
'the setter's' becomes 'ive' ('I've', referring to the crossword setter).
'posit'+'ive'='POSITIVE'
(Other definitions for positive that I've seen before include "Confident, very sure" , "Sure, convinced" , "'Definite, categorical (8)'" , "Definite, sure" , "Of a number, greater than zero - that's certain" .)