Output clock of the LPC1768?
According to this schematic, the pin you are using for CLKOUT is P1.27, which is used for the Ethernet clock enable (ETH_OSC_EN). This is pin 43 on the LPC1768 chip and goes to the enable pin on the Ethernet oscillator (ASE-50-C-T). It does not appear to be broken out anywhere else on the board, so you will have to solder a wire to it at one of those two points on the board.

If you need to generate an external clock without using CLKOUT and without wasting CPU cycles, you can use one of the match outputs. Set the match register to some small value (which will be your clock divider from your PCLK) and enable the corresponding counter/match register. Set it to reset the counter and toggle the output on a timer match event.