The lifelong disparity between appearance and reality runs throughout The Great Gatsby as Gatsby tries to be what he is not and Daisy is certainly not all that Gatsby imagines her to be.
Just as Gatsby thinks that he is about to attain his lifelong dream of a life with Daisy - even though she is married to Tom Buchanan, the reader is reminded of all the time wasted in the process of acquiring the wealth which Gatsby believes will win Daisy over this time around; his stint in the army and lack of material wealth keeping Daisy out of his reach previously.
The clock which Gatsby leans against, but catches before it falls to the ground, is broken and, figuratively, Gatsby's catching the clock represents his efforts to regain a time that he has lived to repeat, especially now that he is in a position to provide Daisy with everything her heart desires. "He turned and caught it with trembling fingers and set it back in place." Unfortunately, time cannot be replaced in the same fashion.
The clock is also symbolic because, whilst Gatsby thinks he now has a chance with Daisy and effectively the years and his dealings do not matter, time will eventually catch up with him and he will be betrayed and murdered for Daisy's crime. His funeral will be poorly attended and time will have run out!