Should be zero? (5)

I believe the answer is:

ought

I believe this is a double definition.

'should be' is the first definition.
(I've seen this before)

'zero?' is the second definition.
(I've seen this in another clue)

(Other definitions for ought that I've seen before include "Be bound" , "Be logically necessary" , "Tough (anag)" , "Have/has a duty (to do)" , "Anything at all (archaic)" .)

I've seen this clue in The Globe and Mail.

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