Mantelpiece Clock emoji is the picture of a decorative type of clock, which typically stands on the shelf over the fireplace in the classic interiors (or elsewhere). In its form, it is a bit similar to the clock represented by ⏰ Alarm Clock emoji and usually, it is also mechanical. As for meanings, the clock is a symbol of time so the emoji may mean anything related to it.
Examples of using
“It’s amazing how quickly years pass by 🕰️”
“The time is running out 🕰️”