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