Deny that over four are not positive (8)
I believe the answer is:
negative
'not positive' is the definition.
('negative' is the opposite to 'positive')
'deny that over four' is the wordplay.
'deny' becomes 'negate' (I've seen this before).
'that over' means one lot of letters goes inside another.
'four' becomes 'iv' (roman numeral).
'negate' going around 'iv' is 'NEGATIVE'.
'are' acts as a link.
(Other definitions for negative that I've seen before include "Expressing or implying denial or refusal" , "The photographic film expresses refusal" , "Snap out of it? Yes" , "Opposite of affirmative" , "veto" .)