"clock" is a generic term in digital logic and can mean different things depending on the application. In your instance the clock for the serial led drivers comes from a port pin that your software toggles to shift out the serial data from another port pin. Put simply, you would have two port pins that we might call 'clock' and 'data'. Your software would output the data bit on 'data' then toggle 'clock' to shift it into the slave device, this repeats until you have shifted the required number of data bits to the slave device. You might have other signals like 'load' and/or 'select' depending on the actual slave device.