Hi Julian,
To use the internal clock of the ADS1299, the CLK_SEL pin must be tied high to a logic "1," either from a microcontroller GPIO or in hardware through a resistor. Depending on whether you want to use the internal clock elsewhere in your system, you may choose to output it though the CLK pin by setting the CLK_EN bit in the CONFIG1 register to 1. Otherwise, this bit may be set to 0, which would place the CLK pin in a 3-state status.
Regards,