Single cryptic crossword clue: one beer... [8]

This is a single cryptic crossword clue. The clue is the clue and not this preamble.

One beer at German festival for relative genius [8]

Answer

This is hardly cryptic, if you know how British crosswords work...

Einstein

"German festival" suggests beer steins, a common sight, and more-or-less a trope to non-Germans. "One beer" would be "ein Bier" in German, but it could also be "one stein (of beer)", or "ein Stein". The "relative genius" clinches it; Albert Einstein was unquestionably a genius, and is best known for his Theory of Relativity.