Obligation on American (4)

I believe the answer is:

onus

'obligation' is the definition.
(thesaurus)

'on american' is the wordplay.
'american' becomes 'us'.
'on'+'us'='ONUS'

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

I've seen this clue in The Times.

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