Taj Mahal topper (5,4)
(Other definitions for onion dome that I've seen before include "part of Russian church?" , "Byzantine structure?" , "Roof feature such as on Brighton Pavilion" , "Roof shaped like a pungent vegetable, on a Russian church, perhaps" , "exotic church feature" .)
'taj mahal topper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dome that I've seen before include "millennium feature" , "Cupola - millennium building" , "Crown" , "Old cathedral" , "Rounded vault of a roof" .)