Charge fine car up keeping close to motor (6)
I believe the answer is:
tariff
'charge' is the definition.
(I know that tariff is a more specific form of the action tax)
'fine car up keeping close to motor' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'car' becomes 'fiat' (car manufacturer).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'close to' suggests the final letters (the close/ending of).
The final letter of 'motor' is 'r'.
'f'+'fiat'='ffiat'
'ffiat' written backwards gives 'taiff'.
'taiff' placed around 'r' is 'TARIFF'.
(Other definitions for tariff that I've seen before include "List of prices or charges" , "Price list - import tax" , "Import/export tax" , "List of duties on imports or exports" , "Bill of fare" .)