A presence which times the time of freedom that Stroud has while at the same time measuring the amount of time that he does not have to prove his innocent.
The big clock therefore, has been used to represent the thread which ties all of us down and which does not allow man the freedom that he so rightly desires. The rhythm of the clock represents the marching tone to which we all have to march irrelevant of whatever we actually think about the tone itself.
Even Stroud finds himself losing control over this very rhythm as he sees time slipping by while he is simply standing helpless on the side. He is unable to deny his involvement as that very denial becomes the cause for his downfall. Even though Stroud does consider himself, to be a man who makes his own decisions and a free man in that sense, he is equally tied as us all.
The pincher claws and the grinding gears of the big clock have caught Stroud in their midst and from him there seems to be no hope. Hence, the movie represents vital elements of the media industry while at the same portraying the extent to which a man can be beholden to circumstances created by himself and by circumstances themselves.