The definition of bragging about having a big dick but really having a small one.
Another meaning for Clayten is one who thinks everyone else is stupid but they are really the stupid one.
Girl: Wow I bet he has a huge cock!
Guy: Nah hes just a Clayten
Kid: Hahaha you only got one mark higher then me on the test stupid!
Other kid: Shut up, your just a Clayten