About United States’ responsibility... (4)
I believe the answer is:
onus
'responsibility' is the definition.
(I've seen this before)
'about united states'' is the wordplay.
'about' becomes 'on'.
'united states'' becomes 'us' ('US' can be a synonym of 'united states').
'on'+'us'='ONUS'
(Other definitions for onus that I've seen before include "'Burden, of proof (4)'" , "'Responsibility, task or burden (4)'" , "Weight of responsibility" , "Burden, duty" , "'Burden, load (4)'" .)