State of half optimism (5)
I believe the answer is:
'state' is the definition.
(I know that state can be written as posit)
'half optimism' is the wordplay.
'half' indicates taking half.
'optimism' becomes 'positivity' (a positive outlook).
'positivity' with half the letters taken is 'POSIT'.
'of' acts as a link.
(Other definitions for posit that I've seen before include "Put forward (as fact)" , "Assume as fact for argument's sake" , "Assume as fact, postulate" , "Put forward (something) as a given" , "Assume something as a basis for discussion" .)