I've got to the third the "don't take it literally" interpretation. I also have issues with my mind racing, but I've been practicing yoga and what I call "hardcore unitasking" for the past two years and I've noticed some changes in how my mind moves.
First of all "clear your mind" is a goal, but it's not the kind of goal you achieve, it's one you strive for knowing that perfection will always elude you. It's a kind of Zen thing. But just saying, "Yo! Clear your mind!" isn't helpful to anyone who isn't already a practitioner because how on earth do you do that?!? Well, it's basically a catchphrase for "Pick some kind of meditation technique or meditative activity that will slow down your thoughts and allow you to observe them and your present." Your daughter in law is probably just failing at explaining what she means. My guess is your son's constant busyness is making her uncomfortable and she wants him to be more like her and this is her way of expressing it -- by saying that he needs to change. That's a normal thing for people to do, although I don't approve of making your problems about other people.
Hardcore unitasking, btw, is my own term for when I pick an activity and force myself to stay with it and not respond to my mind pulling me in every direction until I'm completely finished with that activity. I usually make tea like this when I'm feeling overwhelmed. I just stand there and watch the pot boil. Then I sit there and watch the steam rise until my tea is steeped. No phone. No tidying. No anything but the tea until it's gone. I came up with this idea when I thought I only had ADHD. I found that more than anything it helps me to get a third person perspective on all the noise in my head. The noise doesn't go away, but I can see it for what it is: noise, and not feel like I have to respond to every alarm going off. It's helpful.
Anyway,
@Pats , I just got diagnosed with ASD and I just joined this forum all about a week and a half ago. I hope that I'm not out of line in anything I said here. I felt incredibly welcomed by this community and I would really like to finally have a tribe of my own, so please let me know if I was off target on anything. Your post, and your feeling of exasperation with your daughter in law is very similar to how I've reacted to some of the nonsensical bogus things people have said to me in the past and I'm hoping that means that maybe we operate on the same wavelength? It would be nice to have anyone at all operating on the same wavelength as myself. You know, so we could be like, "yeah, isn't that absurd!" but, you know, together.