Not certain (8)
'not certain' is the definition.
('unsure' is the opposite to 'certain')
This is all the clue.
(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" .)