Imagining (8)
'imagining' is the definition.
(dreaming is a kind of imagining)
This is the entire clue.
(Other definitions for dreaming that I've seen before include "Having fantasies in mid-range" , "Fantasising" , "'I'm ... of a white Christmas' (song)" , "Having fantasies about Niger Dam" , "form of idealism?" .)
(Other definitions for ideation that I've seen before include "Power of the mind for forming ideas/images" , "Power of forming images" .)