College study describing king's mythical creature (7)
I believe the answer is:
'mythical creature' is the definition.
(I've seen this before)
'college study describing king's' is the wordplay.
'college' becomes 'uni' (short for university).
'study' becomes 'con' (conning is a kind of studying).
'describing' indicates putting letters inside (describe can mean to form the outline of).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'con' placed around 'r' is 'corn'.
(Other definitions for unicorn that I've seen before include "Mythical one-horned creature", "Mythical beast like horned horse", "Heraldic beast on the Royal Arms", "fabulous thing to behold", "Imaginary creature with one horn".)