Where the responsibility lies (4)

I believe the answer is:

onus

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'where the responsibility lies' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

This is all the clue.

Can you help me to learn more?

(Other definitions for onus that I've seen before include "albatross?" , "Capulet opponent (R&J)" , "'Responsibility, task or burden (4)'" , "Responsibilty" , "'Burden, of proof (4)'" .)

I've seen this clue in The Irish News.

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