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 default shape of a grandfather clock, thanks to its functional chameleon circuit. This particular shape was one of many default appearances used by the Master's TARDIS. (TV: The Deadly Assassin, The Keeper of Traken, AUDIO: And You Will Obey Me, The Two Masters; PROSE: The Eight Doctors)
John Quinn had a grandfather clock in his cottage at Wenley Moor. (TV: Doctor Who and the Silurians)
In the Edwardian house Edward Grove, the chimes of a grandfather clock could be heard every time the time loop started over again. (AUDIO: The Chimes of Midnight)
Bilis Manger hid an important part of the rift manipulator in a grandfather clock in his office. (TV: Captain Jack Harkness)
The Queen of Time intended to turn the Second Doctor into a grandfather clock at the end of her games. (AUDIO: The Queen of Time)
Missy took a grandfather clock from the Davis household. (AUDIO: A Spoonful of Mayhem)
By 2254, several buildings in London had grandfather clocks within them. (GAME: Dalek Attack)