Genuine Frenchman in kingdom (5)
I believe the answer is:
realm
'kingdom' is the definition.
(kingdom is a type of country or realm)
'genuine frenchman' is the wordplay.
'genuine' becomes 'real' (similar in meaning).
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'real'+'m'='REALM'
'in' is the link.
(Other definitions for realm that I've seen before include "Kingdom could be lamer" , "Area of influence" , "field of study" , "monarch's responsibility" , "Kingdom, domain" .)