like budhy said, without crystal, yes you can have something made if you ensure that a clock like signal is still generated and provided... and that it is steady...
however its really not possible to have an embedded system without a clock...
infact i would say that someting that runs without a clock is not really an embedded system...
Without a clock embedded system can not be designed. Because clock is main part which controls the various functionalities, scheduling and timers. You can generate a clock signal by a circuitry without a crystal aso but that is also a clock signal. Synchronization is the main thing in embedded system without it system will crash.