Window in a roof (8)
I believe the answer is:
skylight
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">window in a 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 skylight that I've seen before include "Attic window" , "Window in a roof or ceiling" .)