After tenth month, French king got tax (6)
I believe the answer is:
octroi
'tax' is the definition.
(octroi is a kind of tax)
'after tenth month french king' is the wordplay.
'after' means one lot of letters go next to another.
'tenth month' becomes 'oct' (short for October).
'french king' becomes 'roi' (I've seen this before).
'oct' put next to 'roi' is 'OCTROI'.
'got' acts as a link.
(Other definitions for octroi that I've seen before include "Duty on goods entering a town" , "Toll" , "Old tax" , "tax on goods" .)