Motty,
This is currently not available within Issues though it is on the roadmap to allow this in the future. What you need to do is create a Task for the Issue and report time on the Task. We have an App on the Marketplace called "Create a Task From an Issue". This will create a Workflow rule that will do this automatically for you. Here is the link...
http://www.clarizen.com/appsmarketplace/item/Create-a-Task-From-an-Issue.html
Sincerely,
Boris