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