A number expelled are downcast (8)
I believe the answer is:
dejected
'downcast' is the definition.
(similar in meaning)
'a number expelled' is the wordplay.
'a number' becomes 'D' (D is an example).
'expelled' becomes 'ejected' ('eject' can be a synonym of 'expel').
'd'+'ejected'='DEJECTED'
'are' acts as a link.
(Other definitions for dejected that I've seen before include "Depressed and cast down" , "Despondent" , "Glum" , "Melancholy" , "Down in the mouth" .)