Hello admits shocking treatment — celeb initially furious (6)
I believe the answer is:
hectic
'furious' is the definition.
(thesaurus)
'hello admits shocking treatment celeb initially' is the wordplay.
'hello' becomes 'hi' ('hi' can be a synonym of 'hello').
'admits' is an insertion indicator.
'shocking treatment' becomes 'ect' (electro-convulsive therapy).
'initially' indicates taking the first letters.
The first letter of 'celeb' is 'c'.
'hi' placed around 'ect' is 'hecti'.
'hecti'+'c'='HECTIC'
(Other definitions for hectic that I've seen before include "Frantic or feverish" , "'Rushed, madly busy (6)'" , "Agitated and rushed" , "Full of frantic activity" , "Frenzied" .)