German's a mug or a genius (8)
I believe the answer is:
einstein
'a genius' is the definition.
(I've seen this before)
'german's a mug' is the wordplay.
'german's a' becomes 'ein' ('a' in German).
'mug' becomes 'stein' (stein is a kind of mug).
'ein'+'stein'='EINSTEIN'
'or' is the link.
(Other definitions for einstein that I've seen before include "Scientist - seen in it (anag)" , "Albert, who founded the theory of relativity" , "German expert on relativity" , "Iconic theoretical physicist, d. 1955" , "Albert -, famous scientist" .)