Certain to be above zero (8)
I believe the answer is:
positive
'certain' is the definition.
(similar in meaning)
'be above zero' is the wordplay.
I cannot really understand how this works, but
'be' could be 'sit' (sitting is a kind of being) and 'sit' is present in the answer.
'zero' could be 'o' (O resembles 0) and 'o' is found in the answer.
This explanation may well be incorrect...
'to' is the link.
Can you help me to learn more?
(Other definitions for positive that I've seen before include "Definite, sure" , "Optimistic; certain" , "Certain, confident in opinion" , "Constructive, optimistic" , "Of a number, greater than zero - that's certain" .)