Spot for an indoor herb garden (4)
I believe the answer is:
sill
(Other definitions for sill that I've seen before include "Shelf at the foot of a window" , "Ledge beneath a window" , "Underwater rock ledge" , "Horizontal timber forming the bottom of a framework" , "Horizontal intrusion of igneous rock between sedimentaries, as in much of Hadrian's Wall" .)