Tax is included for guitar, if French (6)
I believe the answer is:
tariff
'tax' is the definition.
(I know that tariff is a more specific form of the action tax)
'included for guitar if french' is the wordplay.
'included' says the answer is hidden in the clue (I've seen 'including' mean this).
'TARIFF' is hidden within 'for guitar if french'.
'is' acts as a link.
(Other definitions for tariff that I've seen before include "Government tax on imports" , "Tax or duty on a class of imports" , "List of prices displayed as in pub" , "Import duty, tax" , "List of fixed charges as in hotels" .)