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