Crossword Genius

Responsibility we must shoulder? (4)

Ross

I believe the answer is:

onus

I'm a little stuck... Click here to teach me more about this clue!

'responsibility' is the definition.
(onus is a duty or responsibility)

'we must shoulder?' is the wordplay.
I cannot quite understand how this works, but
'we' could be 'us' and 'us' is present in the answer.
The remaining letters 'on' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for onus that I've seen before include "Burden, duty" , "Responsibilty" , "Weight of responsibility" , "'Burden, load (4)'" , "'Burden, of proof (4)'" .)

I've seen this clue in The Sun.
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