There is an out of the box way of doing this by using one of the many Time Tracking reports provided to you in the Reports Module. The problem is that this will no longer be accurate if you disable the Actual effort updated from timesheet which means future Work Items will not have this enabled and all effort will have to be manually entered at the work item level.
You can create a custom Report but typically Time Tracking Summarizations are not very good with the current Report Module. The good news is that we have a brand new Reports Module coming out February 22nd which will allow you to create more customized reports as well as dashboards. You can also pull the data using the Excel Add-In and use Excel functionality to create the desired report.