Imagined country united after one king, then another, beheaded queen (9)
I believe the answer is:
ruritania
'imagined country' is the definition.
(I know this)
'united after one king then another beheaded queen' is the wordplay.
'united after one king then' becomes 'ru' (I can't explain this - if you can you should believe this answer much more).
'another' becomes 'r' (I've seen this in other clues).
'beheaded' means to remove the first letter (the head of the word is removed).
'queen' becomes 'titania' (fairy queen).
'titania' with its first letter taken off is 'itania'.
'ru'+'r'+'itania'='RURITANIA'
(Other definitions for ruritania that I've seen before include "adventurous setting" , "romantic setting" , "mythical land" , "Hope place" , "Hope country" .)