How to define clock while using flip flop

I am new to VHDL and i am facing one problem while writing a code. In my circuit i have different components like half adder, and gate and flip flop. When i perform the check syntax, i receive an error that clock is not defined. Attached is my code, please have a look on it and let me know my mistake. I know this question might looks stupid to some of you but as i am beginner so i would appreciate your feedback.

Thanks,

Syed Irtaza Haider