The first step is to be clear with your employer about your plans. If he gave you the green light, then read the following lines. If he did not give you green light (a reference for other readers of this topic), then you should visit a legal advisor to review your contracts. Employers are not always right and fair.
Now, you're asking how to approach to your future clients. Unfortunately, this is a two-bladed sword. One client will appreciate your honesty while other will run away from you.
So the first step is to understand that you will not be able to work as much as you think. You may be too tired on some days, you may have family issues, you may get sick, etc. So if your main work week takes 40 work hours, the realistic numbers are between 10 and 20 work hours that you can count on every week. And I am talking from my own experience as a freelancer and as an employer.
After you realized your work capacity, you should approach to each project individually.
For example, for projects that you can finish in 1 month, I would not tell clients anything about my main job. These are the jobs you can finish in approx 60-80 work hours.
For larger jobs, you can do some of the following:
increase time frame and decrease the price. most of my clients like this, except those who are in urgency
find your own contractor and outsource projects to them. you should know that by hiring someone else to do the job for you, you will not be able to sit in a chair and enjoy. you can trust me that you will spend all your work hours working on this project as well by planning it, monitoring it, pushing a contractor to work faster, etc.
I would avoid urgent jobs if you cannot handle them yourself. All my mistakes came from taking urgent jobs and hired someone else to finish it. Baad mistakes of mine.
if you take a large project, then you will either have to hire a team full time until the project is finished, or find a good remote team (who costs) and decrease your earning to minimum. I also avoid such project if my own team cannot handle it in-house.
In the end, do not be too honest with clients. Most of them are scared and will decline you quickly. Those who don't decline you probably want much from you for little money. The only thing where you MUST be totally honest is time frame and costs and a knowledge that you can actually finish the job. Getting the job is most times easier than finishing it.