The burden is on America (4)
I believe the answer is:
onus
'the burden' is the definition.
(synonyms)
'on america' is the wordplay.
'america' becomes 'us' (United States).
'on'+'us'='ONUS'
'is' acts as a link.
(Other definitions for onus that I've seen before include "'Burden, of proof (4)'" , "Encumbrance" , "albatross?" , "Liability" , "Obligation - charge" .)