Demo about cupola (4)
I believe the answer is:
dome
'cupola' is the definition.
(architectural term)
'demo about' is the wordplay.
'about' indicates anagramming the letters.
'demo' with letters rearranged gives 'DOME'.
(Other definitions for dome that I've seen before include "description of church roof?" , "Cupola - millennium building" , "Hemispherical structure" , "Rounded roof with circular base" , "millennium feature" .)