Tax is a chore (4)
I believe the answer is:
duty
I believe this clue is a double definition.
'tax' is the first definition.
(duty can mean a tax or toll)
'chore' is the second definition.
(I've seen this in another clue)
'is a' acts as a link.
(Other definitions for duty that I've seen before include "assignment" , "Is one bound" , "A tax; compulsory task" , "Government tax" , "Not subject to customs tax" .)