Yup, we clean up before we eat (so mid morning,and before lunch, and later afternoon).
I have a couple of kids that refuse, or just look at me strangely. You can tell who gets taught to clean up at home!!
I think it can be overwhelming if we just say "Clean it!"
I will give them one block or car, and tell them (or show them) exactly where it goes. If need be, I will gently lead them there. We make it a game sometimes, and see who can go "faster, faster, FASTER!!". There is also a clean up song to sing. I know that in my son's kindergarten class, the teacher had the kids to the point that all she had to do was sing the song and they started to tidy up.