5v clock feeding 3.3v microcontroller - resistor+zener acceptable?
If capacitance (track + diode+ load) is 30pF at 0V with 10k the rise time ~ 300ns in a 50 ns half cycle.  Even a Rohm low capacitance zener=3pF + 3pF optimal load is 60 ns rise time.   No good.  Use an active level converter.