A grandfather clock was a large clock that was approximately the height of an average human male. Its size meant that it had to rest on the floor, rather than upon a mantle piece or bookshelf. Its larger size meant that it had a much louder, deeper chime than the typical mantelpiece clock.
At times, the Master's TARDIS took the shape of a grandfather clock, thanks to its functional chameleon circuit. (DW: The Deadly Assassin/The Keeper of Traken)
In the Edwardian house Edward Grove, the chimes of a grandfather clock could be heard every time the time loop started over again. (BFA: The Chimes of Midnight)
Bilis Manger hid an important part of the Rift manipulator in a grandfather clock in his office. (TW: Captain Jack Harkness)