Responsibility is ours (4)

I believe the answer is:

onus

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'responsibility' is the definition.
(onus is a duty or responsibility)

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

Can you help me to learn more?

(Other definitions for onus that I've seen before include "'Burden, of proof (4)'" , "Weight of responsibility" , "Obligation - charge" , "Capulet opponent (R&J)" , "'Responsibility, task or burden (4)'" .)

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

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