High Speed Digital Signal Measurement
There are three requirements for a 10:1 passive probe to work correctly

The oscilloscope must be used on the 1 MΩ input
The tip ground of the probe must be connected by a short wire to the ground on the circuit local to the clock driver
The probe must be tuned to the oscilloscope's input capacitance

Although the resistance of a 1 MΩ scope input is well standardised, the input capacitance can vary between brands, usually in the range 15 pF to 30 pF. Most 10:1 probes have an adjustable capacitor in parallel with their internal 9 MΩ resistor, which needs to be adjusted to get a good squarewave on the scope. Many scopes include a low frequency squarewave output, usually marked 'calibrator' for this purpose.
As an alternative, you can build a very low capacitance probe to use with your scope's 50 Ω input. Use a series resistor of 500 Ω to 5 kΩ, depending on the load your driver is happy to tolerate, into a 50 Ω cable going to the 50 Ω scope input. As before, ground the business end of the 50 Ω cable to a ground local to your driver. For the resistors I've mentioned, you'll get an 11:1 to 100:1 attenuation.
Most RF and microwave engineers have several of these little probes kicking around to solder onto circuits, lose or break. They are very cheap to build, and very good performance to GHz, if you keep your connections short.
Question for bonus marks. We've all heard about matching. Why is using a 5 kΩ resistor in series with a 50 Ω line OK in this context.