Duty or 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.
(synonyms)
(Other definitions for onus that I've seen before include "albatross?" , "Obligation - charge" , "Encumbrance" , "Capulet opponent (R&J)" , "'Responsibility, task or burden (4)'" .)