Duty — and where so often it seems to lie? (4)
I believe the answer is:
onus
'duty' is the definition.
(I've seen this before)
'and where so often it seems to lie?' is the wordplay.
I cannot quite understand how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
The remaining letters 'ous' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for onus that I've seen before include "Burden, duty" , "albatross?" , "Encumbrance" , "'Responsibility, task or burden (4)'" , "Liability" .)