Fanciful plan (4,5)
I believe the answer is:
pipe dream
'fanciful plan' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pipe dream that I've seen before include "Fantastic but vain hope" , "Unattainable hope" , "Sherlock's flight of fancy" , "Fantastic notion!" , ""Pie in the sky"" .)