You can use one of the PWM pins on Arduino to output a PWM signal. If you want a constant clock, you need to set the duty cycle of the PWM to be 0.5, i.e. 50%.
Syntax: analogWrite(pin, value) where the parameter "value" is the duty cycle ranges from 0 (always off) to 255 (always on) since it is a 8-bit PWM generator inside Arduino.
If you need a PWM wave with duty cycle of 0.5, you need to set the "value" above to be 127, which is exactly in middle of 0 and 255:
analogWrite(clkpin, 127);