A French kingdom, not entirely fantastic? (6)
I believe the answer is:
unreal
'fantastic?' is the definition.
(fantastic can mean unreal or imaginary)
'a french kingdom not entirely' is the wordplay.
'a french' becomes 'un' ('a' in French).
'kingdom' becomes 'realm' (kingdom is a type of country or realm).
'not entirely' means to remove the last letter (not all of the word).
'realm' with its final letter taken away is 'real'.
'un'+'real'='UNREAL'
(Other definitions for unreal that I've seen before include "Phantasmagorical" , "Dreamlike" , "Lacking in substance" , "Illusory, not actual" , "Not actually existing" .)