Burden, duty (4)
I believe the answer is:
onus
I believe this clue is a double definition.
'burden' is the first definition.
(synonyms)
'duty' is the second definition.
(I've seen this in another clue)
(Other definitions for onus that I've seen before include "Duty or responsibility" , "Responsibilty" , "'Responsibility, task or burden (4)'" , "Burden, duty" , "'Burden, load (4)'" .)