Sill (5)
I believe the answer is:
ledge
'sill' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)