Genuine Frenchman in field (5)
I believe the answer is:
realm
'field' is the definition.
(I know that realm is a type of field)
'genuine frenchman' is the wordplay.
'genuine' becomes 'real' (similar in meaning).
'frenchman' becomes 'M' (abbreviation for Monsieur - like 'Mr').
'real'+'m'='REALM'
'in' acts as a link.
(Other definitions for realm that I've seen before include "Kingdom; field of interest" , "Area of influence" , "monarch's responsibility" , "Province" , "Kingdom's" .)