Gloomy about one’s son’s expulsion (9)
I believe the answer is:
dismissal
'expulsion' is the definition.
(thesaurus)
'gloomy about one's son's' is the wordplay.
'gloomy' becomes 'dismal' (synonyms).
'about' indicates putting letters inside.
'one's' becomes 'is' (Roman numeral).
'is'+'s'='iss'
'dismal' enclosing 'iss' is 'DISMISSAL'.
(Other definitions for dismissal that I've seen before include "Notice" , "removal from office" , "brush-off" , "Process of being sent away" , "ending" .)