Should be 0 - N (5)
I believe the answer is:
ought
'should be' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
(Other definitions for ought that I've seen before include "Should, had, better" , "Strange thing" , "outdated anything at all?" , "Be bound" , "Verb implying duty; zero, nothing (archaic)" .)