Spell of work one is obliged to do (4)
I believe the answer is:
duty
'spell of work' is the definition.
'duty' can be an answer for 'work' (duty is a kind of work). I am not sure about the 'spell of' bit.
'one is obliged to do' is the wordplay.
I cannot really understand how this works, but
'do' could be 'ut' (synonyms) and 'ut' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for duty that I've seen before include "Moral or legal obligation" , "Not subject to customs tax" , "Is one bound" , "Task; military service" , "assignment" .)