One may be bound to do it (4)
I believe the answer is:
duty
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'one may be bound to do' is the wordplay.
'one may' becomes 'dy' (I can't explain this - if you can you should give a lot more credence to this answer).
'be bound' is an insertion indicator (some letters are bound in others).
'to do' becomes 'ut' (both can mean the first note in a musical scale).
'dy' enclosing 'ut' is 'DUTY'.
Can you help me to learn more?
(Other definitions for duty that I've seen before include "Moral or legal obligation" , "Is one bound" , "Obligatory task(s) - tax" , "form of tax" , "Service - tax - office" .)