Certain; optimistic (8)
I believe the answer is:
positive
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">certain</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">optimistic</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(associated in meaning)</span><br/ ><br/ ></span>
(Other definitions for positive that I've seen before include "'Definite, categorical (8)'" , "Sure, convinced" , "Optimistic; certain" , "Constructive, optimistic" , "Very sure, certain" .)