Not positive (8)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not positive</span>' is the definition.<br/ ><span class="explanation-format__explanation">('negative' is the opposite to 'positive')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for negative that I've seen before include "Expressing or implying denial or refusal" , "Piece of photographic film" , "Pessimistic, discouraging" , "This required for development" , "Less than zero quantity" .)
(Other definitions for uncertain that I've seen before include "Doubtful, indecisive" , "dubious" , "Not definite" , "Unsure" , "up in the air" .)