It is very important to shield clock lines because:
1)It may couple to other sensitive analog signals
2)Other switching signal may couple to it causing jitter on it.
To shield clock :
1)Shield it with VDD on one side and GND on other side.This helps to provide return current path for magnetic flux lines.Traditional shielding with GND on both sides only helps from reducing influence of coupling capacitor perspective
2)Ideally if there is space you could consider enclosing the clock signal in a wall kind of structure, but this would lead to lot of capacitance loading on teh clock signal and hence loading.