Duty, burden (4)
I believe the answer is:
onus
I believe this is a double definition.
'duty' is the first definition.
(I've seen this before)
'burden' is the second definition.
(I know that burden can be written as onus)
(Other definitions for onus that I've seen before include "Encumbrance" , "albatross?" , "Capulet opponent (R&J)" , "Responsibilty" , "Weight of responsibility" .)