how to view clock in signal tap analyzer?

Is there a reason why you want to see the clock? Do you suspect the clock to be problematic? There is typically no point in seeing the clock. The samples captured in SignalTap are sampled on the clock. So each sample is a clock edge. If the clock isn't running, you won't get any samples and signaltap will just hang waiting for samples when you click the capture button.

Jake

Signaltap is based on sampling signals on a fast enough sampling clk.

To view your clk then you need a faster clk to sample it without violating the sampling theorem. Moreover to see your clk duty cycle reasonably represented then the sampling clk must be high enough to track the transitions of your test clk. At the end you can't really depend on signaltapfor phase testing.