Imaginary being (3)
I believe the answer is:
elf
'imaginary being' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for elf that I've seen before include "A leprechaun" , "Hobgoblin" , "Supernatural creature of folk tales" , "Fairy-like creature" , "Malignant fairy" .)