Tax obligation (4)

I believe the answer is:

duty

I believe this clue is a double definition.

'tax' is the first definition.
(duty is a kind of tax)

'obligation' is the second definition.
('duty' can be a synonym of 'obligation')

(Other definitions for duty that I've seen before include "Official obligation" , "'Obligation, responsibility (4)'" , "charge" , "Service - tax - office" , "Tax - burden" .)

I've seen this clue in The Irish Times, The Independent and The Guardian.

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