A clock is a mechanism that tells the time of day. It’s a period artifact, but no period heraldic examples of its use have been found. (Its earliest known use as a charge is in 17th C. armory.) The illustration is of the type known as a “chamber clock”, mid-16th C. [Ernest Edwardes, Weight-Driven Chamber Clocks of the Middle Ages and Renaissance, plates 18-19; cf. Amman 75].
There is also an example in Society armory of the “clockface”, with the hours (twelve by default) marked around the edge of a roundel. The use of the clockface alone is deemed a step from period practice. For related charges, see astrolabe, equatorium, sphere, sundial.
John Gal of Freeston bears: Argent, a chamber clock azure faced argent.
Lillian Taylor bears: Azure, on a round clockface argent numbered sable a lizard tergiant bendwise vert, all within a bordure argent ermined azure.