Burden on America (4)
I believe the answer is:
onus
'burden' is the definition.
(I know that burden can be written as onus)
'on america' is the wordplay.
'america' becomes 'us' (United States).
'on'+'us'='ONUS'
(Other definitions for onus that I've seen before include "'Responsibility, task or burden (4)'" , "Weight of responsibility" , "Responsibilty" , "Liability" , "Capulet opponent (R&J)" .)