Scatterbrain (7)
'scatterbrain' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for airhead that I've seen before include "Stupid person" , "Vacuous person" , "clown" , "Fool" , "one with a small brain" .)
(Other definitions for space cadet that I've seen before include "one who often gets high" , "trainee astronaut" , "one high on drugs" .)