The clock in this story is symbolic of mortality. It is made of ebony, a black wood, and black is often symbolic of death. Further, the clock is housed in the seventh, westernmost room in Prince Prospero's abbey, a room of black and "blood red." Red, in this story, is also associated with death because it is the "Avatar and [...] seal" of the terrible and fatal disease raging through the kingdom. Moreover, the sun's route is frequently symbolic of a human lifespan: sunrise is birth, the sun at its zenith is midlife, and the sunset is death. The sun rises in the east and sets in the west, and the room farthest to the west is the room made up of colors associated with death. For these reasons, in part, the clock seems to be symbolic of mortality.
In addition, the clock has a fearful effect on the partygoers. When the revelers hear its chimes, and most especially the chimes at midnight, even "the giddiest grew pale" and the older members of the company "passed their hands over their brows as if in confused reverie [...]." The clock makes them nervous, as if it reminds them of the inevitable passage of time and their own limited amount of it. They have tried to escape death by coming to the abbey, but the clock serves as a reminder that they cannot live forever (and this is another reason that they avoid the room where the clock is housed).
In the end, after the Red Death has made its way through the company, "the life of the ebony clock went out with that of the last of the gay." Even the personification of the clock as possessing a life that can "[go] out" seems to confirm the idea that the clock symbolizes mortality. When everyone present is dead, it goes dead too.
The clock, which signifies the passage of time, acts as a foreboding messenger of time. Set in the "western or black chamber"--west is the direction of the sunset and is symbolic of the end of life; black, of course, is the color of the shrouds of death--the tolling of the clock marks the passage of time and the approach of the end of life.
Each time the ebony clock tolls, the guests of Prince Prospero become pale and the oldest ones wipe their foreheads that have dampened with nervousness. Even the musicians are required to pause because the chiming is so loud. Finally, the ebony clock strikes midnight and there enters into the minds of the guests a dreadful meditation. Suddenly, there are those who become aware of a masked figure and the guests begin to murmur and feel terror. It is then that the terrible figure of the Red Death appears and the enraged Propero accosts him only to meet his end "within the shadow of the ebony clock."
Traditionally, clocks often represent time or the passage of time. Poe uses the clock in this manner. The ebony or black clock stands in the last room of Prince Prospero's castle. Black is a traditional symbol of death. If you think of each room as representing a different stage of life, then the black clock in the last room has to symbolize death or time left until death. At a very basic level, the dancers understand this. After all, they came to the castle so they could escape death by shutting out all other people. But surely they knew they could not escape a natural process forever. Thus, they stop when the ebony clock chimes because it reminds them that their time is limited. That is why when the red death finally makes an appearance, he goes to the last room, the symbol of death, where eventually all the revelers die.
The ebony clock is a symbol. A symbol is an object, name, person, etc., that represents something more than its literal meaning.
First, ebony is the color black. The color black is often associated with death. This is symbolic because the party-goers die. Second, a clock is associated with time, obviously. The clock is representative of the borrowed time that the party-goers were living on. Their time to live was ticking away. They had no idea that they would all die that evening.