There is row-level compression and page-level compression (stronger and slower). If you compress clustered index, in fact you compress the table. PK does not have to be the same as clustered key. You should estimate compression gain first to see what you will get in terms of space. Also, be aware that compressing active objects/indexes/partitions is not recommended unless you really have a lot of unused cpu resources. Usually, one compresses old partitions that are queried almost never.