Earl will not participate in adultery, due to moral obligation (4)
I believe the answer is:
duty
'moral obligation' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for duty that I've seen before include "Allegiance" , "Obligatory task(s) - tax" , "Tariff" , "Tax - burden" , "Is one bound" .)