An obligation to produce tax (4)
I believe the answer is:
duty
I believe this clue is a double definition.
'an obligation' is the first definition.
(I know that obligation can be written as duty)
'tax' is the second definition.
(duty can mean a tax or toll)
'to produce' acts as a link.
(Other definitions for duty that I've seen before include "Obligatory task(s) - tax" , "Task; military service" , "Financial or work obligation" , "Tax - burden" , "Allegiance" .)