Nothing in the New Testament is negative (3)
I believe the answer is:
not
'is negative' is the definition.
(I've seen this in another clue)
'nothing in the new testament' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'in the' indicates putting letters inside.
'new testament' becomes 'nt'.
'o' put inside 'nt' is 'NOT'.
(Other definitions for not that I've seen before include "Word of denial" , "Word of negation" , "In no way, never" , "Having close-cut hair" , "Negation of what follows" .)