Optimist's asset (4)
I believe the answer is:
hope
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">optimist's asset</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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'" .)