Regular business tax on goods (6)
I believe the answer is:
custom
This clue could be a double definition.
'regular business' is the first definition.
'tax on goods' is the second definition.
'custom' can be an answer for 'tax' (custom is a kind of tax). I'm unsure of the 'on goods' bit.
Can you help me to learn more?
(Other definitions for custom that I've seen before include "Accepted or habitual practice" , "Usual way of doing something" , "'Habit, practice (6)'" , "Regular trade" , "Traditional habit" .)