I've had loads of issues with seatposts in the past. Some too small, others too wide. Have you tried to fit it, then put the clamp on and tighten it? It may clamp nicely when tightened... If not, try using some thin pieces of aluminium as a shim to tighten it all up. I've used drinks cans before, but the post on my vdc needed so much shimming, that a can wasn't enough. I bought some model makers aluminium sheets on ebay. Got loads left, so will gladly send you some if you like
