The clock tower symbolizes time and progress in Maycomb—especially how slow it is (though certain details do suggest that progress is not static or impossible to attain).
Firstly, we are told that the clock tower is unreliable in telling the time. It moves slowly, much like the attitudes of the people of Maycomb remain much the same as they were during the days of the Old South, when slavery was still legal. These antiquated attitudes include the idea that Black Americans are intellectually and morally inferior to whites and, to a lesser extent, the expectation that girls and women be ladylike.
However, the clock tower was built right before or around the time of the Civil War, and its architecture clashes with the Old Southern style buildings in the area. This suggests there is some progress going on in town; it is mainly engendered in the young, like Scout and Jem, or in older, principled people like Atticus.
So, while progress is slow in coming, there is some hope for the future of Maycomb; though, unfortunately, it is much too late to be of any help to Tom Robinson.