Crossword Genius

Extra cash, not British, becomes burden (4)

Ross

I believe the answer is:

onus

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'burden' is the definition.
(synonyms)

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

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(Other definitions for onus that I've seen before include "'Burden, of proof (4)'" , "Responsibilty" , "'Burden, load (4)'" , "Obligation - charge" , "'Responsibility, task or burden (4)'" .)

I've seen this clue in The Independent.
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