Optimist's asset (4)
I believe the answer is:
hope
'optimist's asset' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hope that I've seen before include "One who wrote about prisoner" , "Feeling of optimism" , "Feeling of expectant desire" , "It alone stayed in Pandora's box" , "'. . . . springs eternal in the human breast'" .)