Star dies unexpectedly -- a calamity (8)
I believe the answer is:
disaster
'a calamity' is the definition.
('disaster' can be a synonym of 'calamity')
'star dies unexpectedly' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'star'+'dies'='stardies'
'stardies' anagrammed gives 'DISASTER'.
(Other definitions for disaster that I've seen before include "Catastrophic occurrence" , "awful situation" , "Star dies (anag)" , "not a success!" , "Great tradegy or cataclysm" .)