Commands like XCLIP and WIPEOUT can work with closed polylines. They are useful commands but unfortunately they work with polylines with straight segments. With the 'Curves to Segment' command you can select linear entities which are curved. The command will trace those entities and creates several small segments where the entity is curved. You can define the size of the segments to increase or decrease the number of segments. If you want the command can delete the original curvy entities from your drawing straight away.