Unrealistic hopes (7,2,3,3)
I believe the answer is:
castles in the air
'unrealistic hopes' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for castles in the air that I've seen before include "rooks circling overhead" , "Fancies" , "Unrealistic hopes" , "daydreams" , "dreamy schemes" .)