Rebuilt atrium is sweet (8)
I believe the answer is:
tiramisu
'sweet' is the definition.
(type of dessert or sweet)
'rebuilt atrium is' is the wordplay.
'rebuilt' indicates an anagram.
'atrium'+'is'='atriumis'
'atriumis' is an anagram of 'TIRAMISU'.
(Other definitions for tiramisu that I've seen before include "Greek dessert" , "Italian sponge dessert" , "Rich dessert, it is a rum (anag.)" , "Dessert soaked in alcohol" , "Arenaceous material" .)