Sid's snapped flower — calamity! (8)
I believe the answer is:
disaster
'calamity' is the definition.
(calamity can mean a disaster)
'sid's snapped flower' is the wordplay.
'snapped' indicates anagramming the letters.
'flower' becomes 'aster ' (type of flower).
'sid' is an anagram of 'dis'.
'dis'+'aster'='DISASTER'
(Other definitions for disaster that I've seen before include "Great tradegy or cataclysm" , "complete failure" , "not a success!" , "Calamity, sudden great misfortune" , "Terrible calamity" .)