It’s in or out of the window (4)

I believe the answer is:

sill

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'window' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for sill that I've seen before include "Underwater rock ledge" , "Mass of rock" , "Shelf at the foot of a window" , "Ledge beneath a window" , "Horizontal intrusion of igneous rock between sedimentaries, as in much of Hadrian's Wall" .)

I've seen this clue in the Evening Standard.

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