Rebuilt atrium is sweet (8)
I believe the answer is:
'sweet' is the definition.
(type of dessert or sweet)
'rebuilt atrium is' is the wordplay.
'rebuilt' indicates an anagram.
'atriumis' is an anagram of 'TIRAMISU'.
(Other definitions for tiramisu that I've seen before include "Italian dessert with coffee and liqueur", "Italian course", "Italian dessert of cake, coffee and chocolate", "Greek dessert", "Dessert -- is it arum? (anag)".)