Nuncio upset about king's mythical creature (7)
I believe the answer is:
unicorn
'mythical creature' is the definition.
(I know that Unicorn is a type of mythical creature)
'nuncio upset about king's' is the wordplay.
'upset' indicates an anagram.
'about' indicates putting letters inside.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'nuncio' anagrammed gives 'unicon'.
'unicon' placed around 'r' is 'UNICORN'.
(Other definitions for unicorn that I've seen before include "Imaginary animal" , "Mythical horned equine" , "Fabled horned beast" , "Imaginary creature with one horn" , "Fabulous animal like horse with horn" .)