Heraldic creature (7)
I believe the answer is:
'heraldic creature' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for unicorn that I've seen before include "Fabulous creature" , "Fabulous one-horned animal" , "Animal rumoured" , "Imaginary creature with one horn" , "Mythical white equine" .)