Understanding Clock Input Signal
I'm not quite sure exactly what you are asking here, but with this type of interface, ideally you need control over both the clock line and the data line, so you can make sure the data line is stable at the edge (this can be rising or falling) of the clock line, which latches the data into the device.
If you look at the diagram below, you can see the data line is stable at the rising edge of the clock. In between transmission, the clock line can stop changing states. Sometimes the initial change of clock line state can be used to indicate the start of a transmission.  



You could use an oscillator, as long as your dataline is syncronised with it, so you can determine when to change states. An external unsyncronised oscllator would make things more complex as you would need to read it's value in order to determine this. Also as Keelan notes, you would not be able to change speeds.