Glazier's replacement

PANE

Think of the flat, transparent piece in a window frame that might get cracked.

A glazier works with glass, often installing or repairing window panes. The clue plays on the idea of replacing something a glazier handles—so instead of the whole window, you’re left with just the "pane," which is the flat sheet of glass. It’s a straightforward definition with a hint of wordplay, since "replacement" suggests swapping out one part (the full window) for another (the pane itself).