...
If you use Clockwork to track work you do on behalf of clients, Clockwork Pro allows you to include a checkbox on your worklogs to indicate if whether or not the work performed is billable.
Use the dropdown menu to enable the billable worklogs. This means that when worklogs are created, there will be a Billable checkbox (checked by default) on every worklog. If you are not using Clockwork to track hours that you will bill, simply disable the feature.
...