Month with French king is duty (6)
I believe the answer is:
octroi
'duty' is the definition.
(octroi is a kind of duty)
'month with french king' is the wordplay.
'month' becomes 'oct' (Oct is an example).
'with' means one lot of letters go next to another.
'french king' becomes 'roi' (I've seen this before).
'oct'+'roi'='OCTROI'
'is' acts as a link.
(Other definitions for octroi that I've seen before include "Toll" , "Old tax" , "tax on goods" , "Duty on goods entering a town" .)