Sill (5)
I believe the answer is:
ledge
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sill</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ledge that I've seen before include "Submerged underwater shelf" , "Shelf-life projection" , "Shelf-like projection" , "A step" , "Narrow projecting shelf, on cliff or building" .)