Wishful thinking (8)
(Other definitions for pipe dream that I've seen before include "Fanciful or impossible plan" , "Sherlock's flight of fancy" , "Fanciful hope" , "Fantastic but vain hope" , "Unattainable hope" .)
'wishful thinking' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for daydream that I've seen before include "Indulge in a fantasy" , "Distracting thoughts" , "(Indulge in) idle reverie" , "Wool-gather" , "Diurnal reveries" .)