Imaginary one-horned creature (7)
I believe the answer is:
unicorn
'imaginary one-horned creature' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for unicorn that I've seen before include "that's fabulous" , "Fabulous animal like horse with horn" , "Singularly horny creature of fantasy" , "Animal of fable" , "Mythical one-horned creature" .)