Help making one movieclip interact with another!

I’m trying to make a flash where im using two mc:s, what I want to achive is that when the first movieclip (lets call it mc1) enters a certain frame a nested movieclip(mc2_1) within the other mc(mc2) dissappears. Mc2_1 should return again if you click on mc2.

Hope you understand what I´m talking about, this would really help me as this is the final thing I need to fix for the project to be completed.