Glass in a frame

PANE

Think of the clear, flat piece you might look through in a window or door—it’s not the whole structure, just one section of it.

The clue "Glass in a frame" points to "pane," which is a flat sheet of glass typically set into a window or door frame. It’s a straightforward definition—no wordplay or tricks here—since "pane" directly refers to the glass component you’d find in framed structures like windows. The spelling might make you think of "pain," but context keeps it simple.