One of the major themes in The Great Gatsby is the nature of time and the past, and we can see this playing out in Chapter 5. Gatsby, awkward and nervous, leans back against Nick's mantelpiece and nearly topples the clock. The symbol of the clock thus also represents one of the novel's themes as Gatsby is nearly knocked on the head by time.
Shortly after this episode, Daisy and Gatsby attempt small talk. Gatsby knows exactly how long it's been since they've spoken: "Five years next November." He (and Daisy) both pretend to ignore the passage of time, as if they can start their lives again where they left off, but yet they are both strangely attuned to the details of time--the exact moment of leaving, the time the tea was supposed to start.
Another major theme in this chapter (and throughout the novel) is the conflict between fantasy/idealism and reality--this is demonstrated through Gatsby's wonder at Daisy and through Daisy's emotional response to Gatsby's "beautiful" shirts. Most celebrated is perhaps the symbol of the green light at the end of the dock--Gatsby's seemingly unattainable dream of Daisy. At the end of the chapter, Nick remarks upon the inevitable collision of dream and reality, and the "colossal vitality of [Gatsby's] illusion."
In chapter 5, Gatsby finally achieves his five-year dream of a reunion with Daisy Buchanan. The theme/conflict is between Gatsby's wish to stop time, to roll back the clock as if the intervening five years never happened, and the reality that time never stops. Gatsby, through force of will and money, wants to restore an imagined perfect past, but the reality of life says that's impossible. Time past collides with time present, dream with reality. As Nick says:
There must have been moments even that afternoon when Daisy tumbled short of his dreams....
The clock on the mantelpiece at Nick's cottage symbolizes the difficulty of trying to control time. The pressure of Gatsby's head causes the clock "to tilt dangerously" and Gatsby catches it as it "falls," showing time's fragility and precariousness. Nick refers to Gatsby himself as an "overwound clock" as he shows Daisy his house and tries to reclaim the past.
Once Daisy is a reality at his side, the green light at the end of the dock, symbolizing Gatsby's unrealized desire to reclaim her, becomes hidden in a "mist." This shows that a dream fades when reality takes over. The light becomes just another green light. As Nick puts it Gatsby's "count of enchanted objects had diminished by one."
While wandering around outside to let Jay and Daisy be alone, Nick looks at Gatsby's house and thinks of a former owner, who also had a dream of going back in time. He wanted the neighboring houses to be thatched roof cottages, as if he were a medieval lord of another era, and like Gatsby, he tried to use money to realize his dream, offering to pay the neighbors' property taxes if they would bend to his will. They wouldn't and the man died, symbolized by the black wreath on his door. Death--marked by the recurring motif of the black wreath in the novel--becomes the ultimate symbol of the impossibility of freezing time.