I'm dead upset by the crown! (6)
I believe the answer is:
diadem
'the crown' is the definition.
('diadem' can be a synonym of 'crown')
'i'm dead upset' is the wordplay.
'upset' indicates an anagram.
'im'+'dead'='imdead'
'imdead' anagrammed gives 'DIADEM'.
'by' acts as a link.
(Other definitions for diadem that I've seen before include "Jewelled headband" , "Royal power" , "Small jewelled crown" , "Ornamental jewelled headdress" , "Kingly crown" .)