"8.9 Clock Output Buffer
The device can output the system clock on the CLKO pin. To enable the output, the CKOUT Fuse has to be programmed. This mode is suitable when the chip clock is used to drive other circuits on the system. The clock also will be output during reset, and the normal operation of I/O pin will be overridden when the fuse is programmed. Any clock source, including the internal RC ?Oscillator, can be selected when the clock is output on CLKO. If the System Clock Prescaler is used, it is the divided system clock that is output."
The CLK0 pin is PORTB pin 0 which on the UNO is known as Pin 8.
You will need an ISP device to set the proper fuse bit. That's bit 6 of the Fuse Low Byte. Set it to 0 to activate the clock output buffer. In boards.txt you would change:
uno.bootloader.low_fuses=0xff
to
uno.bootloader.low_fuses=0xBF
Then re-burn the bootloader. (Part of the bootloader burning is setting the fuses.)