Not certain (6)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not certain</span>' is the definition.<br/ ><span class="explanation-format__explanation">('unsure' is the opposite to 'certain')</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for unsure that I've seen before include "Not certain, dubious" , "Not certain or decided" , "regarded as somewhat precarious" , "Not certain, doubtful" , "self-confidence" .)
(Other definitions for doubtful that I've seen before include "Unsettled in mind or opinion" , "Unlikely, questionable" , "Open to question, uncertain" , "Uncertain, unlikely" , "suspect" .)