To tighten up the freehub body you will need to remove the axle in order to get a very large (10mm-12mm depending upon hub make) allen key in to do so. Usually this is inserted via drive side but some it is from non-drive side. If it is already tight then the little tiny bearings in the freehub body are knackered and you'll need to replace the freehub body - you'd need to identify which fitting it is as there are several different ones.