One German mug? On the contrary! (8)
I believe the answer is:
einstein
'the contrary' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'one german mug? on' is the wordplay.
'one german' becomes 'ein' ('one' in German).
'mug?' becomes 'stein' (stein is a kind of mug).
'on' is a charade indicator (letters next to each other).
'ein'+'stein'='EINSTEIN'
(Other definitions for einstein that I've seen before include "Albert, who founded the theory of relativity" , "Disney's 14 [GREAT DANE]" , "Albert -, Nobel prize scientist" , "Iconic theoretical physicist, d. 1955" , "Father of Relativity" .)