The burden on the United States (4)

I believe the answer is:

onus

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

'on the united states' is the wordplay.
'the united states' becomes 'U.S.' (abbreviation for the United States).
'on'+'us'='ONUS'

(Other definitions for onus that I've seen before include "Weight of responsibility" , "Responsibilty" , "albatross?" , "Burden, duty" , "Obligation - charge" .)

I've seen this clue in the Evening Standard.

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