Nuncio confused about rook as 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 confused about rook' is the wordplay.
'confused' is an anagram indicator.
'about' indicates putting letters inside.
'rook' becomes 'R' (chess abbreviation).
'nuncio' is an anagram of 'unicon'.
'unicon' going around 'r' is 'UNICORN'.
'as' is the link.
(Other definitions for unicorn that I've seen before include "Mythical animal with one horn" , "Imaginary animal" , "Mythical beast like horned horse" , "Animal rumoured" , "Fabulous beast like one-horned horse" .)