Tax is an obligation (4)
I believe the answer is:
duty
I believe this is a double definition.
'tax' is the first definition.
(duty can mean a tax or toll)
'an obligation' is the second definition.
(I know that obligation can be written as duty)
'is' acts as a link.
(Other definitions for duty that I've seen before include "work you are obliged to do" , "Moral/legal obligation" , "Not subject to customs tax" , "Kind of obligation or tax" , "A tax; compulsory task" .)