Monarch initially supporting genuine kingdom (5)
I believe the answer is:
realm
'kingdom' is the definition.
(kingdom is a type of country or realm)
'monarch initially supporting genuine' is the wordplay.
'initially' indicates taking the first letters.
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'genuine' becomes 'real' (similar in meaning).
The initial letter of 'monarch' is 'm'.
'm' after 'real' is 'REALM'.
(Other definitions for realm that I've seen before include "Kingdom could be lamer" , "where ruler holds sway?" , "Peers are possessed by this" , "Royal domain" , "Kingdom's" .)