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 be' becomes 'dy' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'bound' means one lot of letters goes inside another (some letters are bound in others).
'to do' becomes 'ut' (synonyms).
'dy' placed around '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" .)