In reply to Arshia:
As Ben explained, we cant use assertion for generating clock. In this problem, I used assertions to verify my clock.
If you want to generate 25 MHz clock, follow the below instructions.
Convert 25MHz into time period terms.
25 MHz -> 40 ns in time period
As the duty cycle is 50%, the clock changes its value every 20 ns
Use the below code for 25 MHz clock
#20 clock = !clock;