Part-time/ Freelance Work

Where can we check this clause? Will it be separately for big 4 clients?

No. It isn't. do it if it's not a conflict of interest and it doesn't hamper your work. Not from a legal standpoint. But this is more ethical.

Depends on what your contract says. If you want to do it legally of course.

If your contract specifically prevents you from doing so, you are definitely not allowed to do it. Especially if you need to ask.