Sure to be a number greater than zero (8)
I believe the answer is:
positive
'greater than zero' is the definition.
(I've seen this before)
'sure to be a number' is the wordplay.
I cannot really understand how this works, but
'be' could be 'sit' (sitting is a kind of being) and 'sit' is found within the answer.
'number' could be 'iv' (IV is an example) and 'iv' is found within the answer.
The remaining letters 'poe' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for positive that I've seen before include "Certain, confident in opinion" , "Sure, convinced" , "Of a number, greater than zero - that's certain" , "Definite, sure" , "Constructive, optimistic" .)