Impossible dream (7)
I believe the answer is:
chimera
'impossible dream' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for chimera that I've seen before include "Wild or impossible idea" , "Idle fancy" , "A wild fancy" , "Chair me to find mixed monster of Greek myth" , "Mythical monster, part lion, goat and serpent" .)