Obligation to pay a tax (4)
I believe the answer is:
duty
This clue might be a double definition.
'obligation' is the first definition.
(synonyms)
'a tax' is the second definition.
(duty is a kind of tax)
'to pay' acts as a link.
This may not be right. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for duty that I've seen before include "Not subject to customs tax" , "Task; military service" , "Financial or work obligation" , "Tariff" , "assignment" .)