A clock may be constructed to sound a chime on the hour. I suppose in the past, hundreds of years ago, the only clocks struck a gong to announce the time. This usage has remained in the language.

It means that the clock made a audible noise to signify the hour. A striking clock is a clock that sounds the hours audibly on a bell or gong. In 12 hour striking, used most commonly in striking clocks today, the clock strikes once at one a.m., twice at two a.m., continuing in this way up to twelve times at 12 noon, then starts again, striking once at one p.m., twice at two p.m., up to twelve times at 12 midnight. http://en.wikipedia.org/wiki/Striking_clock