... I have a drum track for example, and already played in the hihats. In a second take i want to record the kick and the snare. Record is set to "overdub" , but instead of overdubbing, a second clip gets layerd over the first one in the same track :-( .
 

You want to use a technique Cakewalk calls "sound on sound".  Sound on sound allows new audio to merge with existing audio.  You must select sound on sound as your recording mode.
 
I don't have Music Creator 7 so I can't point you to the documentation.  Here is some information from the knowledge guide you may find helpful:  https://www.cakewalk.com/...help=Recording.11.html

The MC6 and 7 help are also on the web site. The MC7 recording modes page is here https://www.cakewalk.com/Documentation?product=Music%20Creator%207&language=3&help=Recording.10.html#1113237
 

Sound on sound lets you lay the new clip right over the old clip. When you play back, you hear both...or all of the clips if I recall this mode correctly. I never use this mode.  It sounds like this is the mode you are using, so if this is NOT what you want you need to turn it off and use OVERWRITE.
 
I use OVERWRITE and it overwrites the old and replaces it with the new clip.  Quite often I will manually delete the clip first.
 
However, remember that nothing ever really gets deleted. It just gets removed from the project but continues to exist in the cakewalk project folder for that song. 
 
 
If you are recording one part of the kit at a time, many folks will put each new part of the kit on it's own unique track. If you put them all into one track you have less control in the mixing stage. By having them in different tracks.... kick on 1, snare on 2, hats on 3, toms on 4, etc.... you have precise control in the mix.... if the kick needs to come up or down, it doesn't affect the rest of the kit.
 
To do this, you have to assign the mic inputs to the new track for each take. You do this in the Track Inspector for INPUTS.
 
Hope this is useful for you.