One to one relationship or another approach?
To remove extra fields the best approach (In my eye of course) would be to insert site_id to clan,team and game tables.

In the site table we have site_id which is primary key(unique) and we have a field in the name of url. 


  
  url field should be unique.
  one-to-one relationship between your three tables with site table.
  


This is the way to go.