Argument about Arabic customs duty (6)
I believe the answer is:
tariff
'customs duty' is the definition.
(I've seen this before)
'argument about arabic' is the wordplay.
'argument' becomes 'tiff' (I've seen this before).
'about' means one lot of letters goes inside another.
'arabic' becomes 'ar' (abbreviation in Chambers).
'tiff' enclosing 'ar' is 'TARIFF'.
(Other definitions for tariff that I've seen before include "Table of (hotel) charges" , "Export/import duty" , "List of fixed charges as in hotels" , "List of prices or charges" , "Table of fixed charges" .)