Where our responsibility may lie (4)
I believe the answer is:
onus
'where our responsibility may lie' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for onus that I've seen before include "Capulet opponent (R&J)" , "'Responsibility, task or burden (4)'" , "Encumbrance" , "'Burden, of proof (4)'" , "'Burden, load (4)'" .)