Argument about a Republican tax (6)
I believe the answer is:
tariff
'tax' is the definition.
(tariffing is a kind of taxing)
'argument about a republican' is the wordplay.
'argument' becomes 'tiff' (I've seen this before**).
'about' means one lot of letters goes inside another.
'republican' becomes 'r' (abbreviation).
'a'+'r'='ar'
'tiff' going around 'ar' is 'TARIFF'.
(Other definitions for tariff that I've seen before include "Duty; table of charges" , "Tax or duty on a class of imports" , "Price list - import tax" , "Export/import duty" , "Customs-duty" .)