For tasks-based items, the clear button is enabled. The same problem can be seen on both the Timesheets View ( Project Investments Module) and the Resource Timesheets View (Resource Module). Expected to be able to click "Clear" for activity-based items.
A Mariner investment that is task- based , such as a project , can be associated with a Microsoft Project plan. Click the Refresh Plan from Mariner button on the Microsoft Project/Mariner tool bar, or click Refresh Plan from Mariner on the Mariner menu.