Agile Boards
This feature is only available in Clockwork Pro.
Clockwork creates a new custom field called Clockwork Timer that is available to all projects where Clockwork is enabled. If an automatic timer is running (the item is in an active status and has an assignee) then the current amount of time logged against the item will be stored in the field.
You can add this field to cards in your Jira boards to see how much time the item has been spent on the item so far.
To show accumulated time on a Jira board, go to Board settings > Card layout and add the Clockwork Timer custom field.
Note this feature is not available Team-managed projects.