you dont lose land, or ore. The only thing that happens is that instead of having 5 "colonies", you get one "colony" (consisting of the 5 planets).
if you are terran the first 5 (clusterable, so excluding U class planets) planets that are above your homeworld are clustered.
so if you dont like one of them (too small for example) you can plunder it, and the planet above that one would be clustered with the other 4....
they dont have to be of the same metal typ (you selcet one typ of metal to get from the cluster in the clustering process)