Question

how can mcclellan name be traced back to scotland?

Answer 1

Wikipedia has variant names such as MacLellan and McLellan as being of Scottish origin.

https://en.wikipedia.org/wiki/MacLellan_(surname)

See also http://www.ancestry.com/name-origin?surname=mcclellan

Answer 2

Some interesting facts and research about the McClellan name can be found here:

https://www.houseofnames.com/mcclellan-family-crest

https://danielomcclellan.wordpress.com/2010/01/12/what-does-mcclellan-mean/

and here:

http://www.angelfire.com/ut2/dlarsen/manwill/histories/mcclellan/james_mcclellan.html