Delusion (7)
I believe the answer is:
chimera
'delusion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for chimera that I've seen before include "In Greek myth, a monster-mix of lion, goat and serpent" , "fabulous beast" , "Illusory aim; monster" , "wild dream" , "Mythical creature; delusion" .)