It's the map with 6 bases and all the gv spawns in the middle of the map. The name starts with CL something.
I just want to cut the island part out of the map to have for a terrain for a custom arena if that is possible.
(Trying to be helpful here)
You can't do that. You don't have the source files.
Two choices.
You can create a terrain of your own by hand based upon it's clipboard map, and use it in a custom arena, but it'll never the exact same terrain.
You can try to find the creator of the terrain and beg him for the version you envision.
I tried to figure out which terrain you were talking about. The closest I could come to was a ct__ something. That might have been used in the SEA. It's possible it might have been an AvA terrain used for Tank Night, but I doubt that.
If only 6 bases, option #1 isn't out of the question. There's lots of help in the Terrain forum if you decide to go that route.