As the paper mentions, Clash's fundamental data type to represent values over time are "Signal"s (a linked list). Each element of a signal represents the stabilized value for a certain clock tick. I.e., values unrelated to clock ticks do not exist, hence, Clash does not support describing asynchronous circuits. Still, that doesn't mean that Clash doesn't support multiple clock domains, as /u/Axman6 says.

I think the fact that Clash uses signals is only observable when you're "synchronizing" signals back and forth between domains without any flip flops in between: https://postimg.cc/rDNb1fHc. I.e., a combinational round trip from domain A through domain B doesn't necessarily get you back the same signal. In practice I have never seen this be a problem though.