Greater than zero? Sure! (8)
I believe the answer is:
positive
I believe this clue is a double definition.
'greater than zero?' is the first definition.
(I've seen this before)
'sure' is the second definition.
(similar in meaning)
(Other definitions for positive that I've seen before include "Definite, sure" , "Confident, very sure" , "'Definite, categorical (8)'" , "Constructive, optimistic" , "Of a number, greater than zero - that's certain" .)