Nuncio nuts 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 nuts about rook' is the wordplay.
'nuts' indicates anagramming the letters (nuts can mean crazy).
'about' means one lot of letters goes inside another.
'rook' becomes 'R' (chess abbreviation).
'nuncio' with letters rearranged gives 'unicon'.
'unicon' placed around 'r' is 'UNICORN'.
'as' acts as a link.
(Other definitions for unicorn that I've seen before include "Animal rumoured" , "Fabulous animal like horse with horn" , "Team of three" , "Imaginary one-horned creature" , "Fabulous beast like horned horse" .)