Sorry for the late reply. I had just did a search on this and still couldn't find the solution.
Anyways with a little self reseach, I found it to be somewhat of a a little "hidden" fucntion.
But once I found it... WoLLA!!
To merge to clips together:
1. Open the clip you want to merge (clip B)
2. Go to top menu navigation Edit > Add Clip to End...
3. Quicktime will open a window to select the clip you want to merge.
4. Choose that clip (clip A)
Hope this helps