Ormolu clock

The ormolu clock was an ornate clock owned by the Doctor. According to one source, its proper name was a "Jibert Cathcode Troisième timepiece". (PROSE: Cat's Cradle: Time's Crucible)

The Doctor kept the ormolu clock in the TARDIS console room, primarily during his first incarnation. (TV: An Unearthly Child, The Edge of Destruction, The Time Meddler, PROSE: Doctor Who in an Exciting Adventure with the Daleks, Doctor Who and the Crusaders, The Eleventh Tiger, The Chase)

When the fast return switch became stuck, the TARDIS attempted to warn her crew of the danger by melting the face of the ormolu clock, indicating that time was being taken away from them. (TV: The Edge of Destruction)

At some point the Doctor stopped keeping the ormolu clock in the TARDIS console room, but started doing so again in his sixth and seventh incarnations. (PROSE: State of Change, Cat's Cradle: Time's Crucible, White Darkness, Shadowmind, The Death of Art)

The ormolu clock was destroyed when Marnal's fusion reactor detonated inside the TARDIS. (PROSE: The Gallifrey Chronicles)

Other ormolu clocks
The Third Doctor kept an ormolu clock in his watchmender's shop in 1952 London. It was destroyed when Jack "the Lad" Cooper burned down the shop. (PROSE: Amorality Tale)

The Eighth Doctor kept an ormolu clock in his hotel room in 1944 Paris. (PROSE: The Turing Test)

The Doctor of one universe kept an ormolu clock in his rooms on Gallifrey. (PROSE: The Infinity Doctors)

Behind the scenes
The term "ormolu" was never used to describe the clock on-screen, only in prose works, beginning with Doctor Who in an Exciting Adventure with the Daleks. In that novelisation the word is misspelled "ormulu", but it is spelled correctly in every other book except for The Gallifrey Chronicles, its final appearance.