Partly study about tax on goods (4)

I believe the answer is:

duty

I'm a little stuck... Click here to teach me more about this clue!

'tax on goods' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for duty that I've seen before include "'Obligation, responsibility (4)'" , "Moral/legal obligation" , "Levy - tax on imports" , "form of tax" , "Obligatory task(s) - tax" .)

I've seen this clue in the Sydney Morning Herald.

Want a hint initially instead of a full solution? Install my app