One is thus bound to pay it (4)
I believe the answer is:
duty
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for duty that I've seen before include "Levy - tax on imports" , "Tariff" , "Not subject to customs tax" , "Kind of obligation or tax" , "'Obligation, responsibility (4)'" .)