Clock out is only available when using an external oscillator. So if you try this in a wilt an XTAL connected to XTAL1 and XTAL2, the cristal oscillator wont work, and you must use an external oscillator, or the internal oscillator. This may cause problems with serial coms, because it cal deviate +/- 3% !!!
So when using this, remove the Xtal (AFTER PROGRAMMING THE FUSES TO USE THE INTERNAL OSCILLATOR), and only when you are using in external oscillator set the fuses after connecting the external oscillator. To use the SPI programming mode, you must use a working oscillator, so if you remove the Xtal first, you 'll have no working oscillator, and you can't program anything
If you need an oscillator, the FT323RL used on the original NANO can be programmed to produce an 12 MHZ clock signal on one of the pins, however this will only work when the USB line is connected, not in standby mode. This is slower than the standard 16 MHZ clock, but will also work with the Nano on 3.3 Volt. You will have to adjust the baudrate of the bootloader accordingly, or recompile the bootloader for 12 MHz.
Also you must add a Arduino Nano @ 12MHZ as a new device, or the compiler will generate incorrect timing intervals, serial speeds and all other timing sensitive items, so this is not advised to be used, but mey be usefull as external clock for programming using the SPI interface