The burden on the Americans (4)

I believe the answer is:

onus

'the burden' is the definition.
('onus' can be a synonym of 'burden')

'on the americans' is the wordplay.
'the americans' becomes 'US' (United States).
'on'+'us'='ONUS'

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

I've seen this clue in the Evening Standard.

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