Hemispherical roof (4)
I believe the answer is:
dome
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hemispherical roof</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for dome that I've seen before include "Crown" , "Curved roof" , "Rounded roof with circular base" , "millennium feature" , "Head" .)