Fabulous sight, drunken nuncio runs inside (7)
I believe the answer is:
unicorn
'fabulous sight' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'drunken nuncio runs inside' is the wordplay.
'drunken' indicates an anagram.
'runs' becomes 'r' (cricket abbreviation).
'inside' means one lot of letters goes inside another.
'nuncio' with letters rearranged gives 'unicon'.
'unicon' enclosing 'r' is 'UNICORN'.
Can you help me to learn more?
(Other definitions for unicorn that I've seen before include "Heraldic creature" , "wonder horse" , "Fabulous beast like one-horned horse" , "Imaginary animal" , "Mythical one-horned creature" .)