Histrionics alarmed Mo terribly (9)
I believe the answer is:
melodrama
'histrionics' is the definition.
(I have seen 'Such histrionics' mean 'melodrama' so perhaps 'histrionics' could also mean 'melodrama')
'alarmed mo terribly' is the wordplay.
'terribly' indicates an anagram.
'alarmed'+'mo'='alarmedmo'
'alarmedmo' with letters rearranged gives 'MELODRAMA'.
(Other definitions for melodrama that I've seen before include "Histrionics - sensational play" , "Sensational theatre piece" , "Tear-jerker" , "Exceptionally sensational play" , "hysterical behaviour" .)