Burden supported by America (4)
I believe the answer is:
onus
'burden' is the definition.
('onus' can be a synonym of 'burden')
'supported by america' is the wordplay.
'supported by' becomes 'on'.
'america' becomes 'us' (abbreviation for the United States).
'on'+'us'='ONUS'
(Other definitions for onus that I've seen before include "Duty or responsibility" , "albatross?" , "Encumbrance" , "'Burden, of proof (4)'" , "Responsibilty" .)