Window ledge (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">window ledge</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 sill that I've seen before include "Window edge" , "Ledge beneath a window" , "Underwater rock ledge" , "Mass of rock" , "Slab at foot of window" .)