College study describing king's mythical creature (7)
I believe the answer is:
unicorn
'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'.
'uni'+'corn'='UNICORN'
(Other definitions for unicorn that I've seen before include "Fabulous horned animal" , "fabulous thing to behold" , "Singularly horny creature of fantasy" , "Fabulous beast like horned horse" , "Mythical one-horned creature" .)