Obliged to be nothing (5)

I believe the answer is:

ought

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'be nothing' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for ought that I've seen before include "outdated anything at all?" , "Strange thing" , "It expresses duty or obligation" , "Anything at all (archaic)" , "Have/has a duty (to do)" .)

I've seen this clue in the Sydney Morning Herald.

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