Burden or duty (4)
I believe the answer is:
onus
I believe this clue is a double definition.
'burden' is the first definition.
(I know that burden can be written as onus)
'duty' is the second definition.
(I've seen this before)
(Other definitions for onus that I've seen before include "'Burden, of proof (4)'" , "Liability" , "Burden, duty" , "Weight of responsibility" , "Responsibilty" .)