Optimistic emotion (4)
I believe the answer is:
hope
'optimistic emotion' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hope that I've seen before include "Trust" , "It goes with faith and charity" , "Optimism - English-born US comedian" , "Feeling of optimism" , "The last thing in Pandora's Box" .)