Indeed the external clock is 10Mhz, You can buy many 'piggyback' PCB's that will fit on the internal connector from the HackRf for the external clock.
But beware that some TCXO pcb have the TCXO on the top of the PCB and the HackRf will not fit it's case anymore, I modified my
TCXO pcb so that it was on the bottom side (same side as the connector) if you don't like soldering look for the right one to order.
Grtzz Mark