Initially forgotten, extra obligation (4)

I believe the answer is:

onus

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'obligation' is the definition.
(I've seen this in another clue)

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 "Weight of responsibility" , "Obligation - charge" , "'Responsibility, task or burden (4)'" , "albatross?" , "'Burden, of proof (4)'" .)

I've seen this clue in The Guardian.

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