If you really look into default values, you quickly realize how few data is actually available on work item creation. The team area is not available, as this needs to be specified by the user after the creation. So default values don't get you that far. Calculated values could be used but have the disadvantage of always being calculated. So you have to be able to distinguish between when to calculate and return another value and when to return the current one.

Then there is the "The team lead changes the team or employer" situation that might need careful consideration.

So the first question would be where that name is supposed to be in - in terms of an attribute. If this is just a string attribute or another additional contributor attribute, you can

1. Create a Java Script calculated value that reads the team area name and has the team lead hard coded or in the configuration
2. Create a Java calculated value that reads the team area and gets the contributor from the team area membership and roles like in https://rsjazz.wordpress.com/2013/06/26/attribute-customization-java-based-value-providers-conditions-and-validators/

Another option is a participant that calculates this on a work item save in the transition to the first state of the workflow.

Unfortunately you lack to explain what the requirement is actually supposed to help with. It would be easy enough to display the team organization in a dashboard which could directly lead to the team where you can see the team member role in a dashboard widget.

I am not aware of an out-of-the-box Team Area Lead Name property, so I assume this is some custom property defined in your project area.   To provide a computed default value such as this, I assume that you will need to write a custom Operation Behavior post-condition on the Save Work Item operation (Ralph's blogs give you guidance on how to do this).