Difficult test checked by German and not disputed (12)
I believe the answer is:
unchallenged
'difficult test checked by german' is the definition.
Although both the answer and definition are adjectives, I don't see how one could define the other.
'not disputed' is the wordplay.
'not' becomes 'un' (prefix meaning 'not').
'disputed' becomes 'challenged' ('challenge' can be a synonym of 'dispute').
'un'+'challenged'='UNCHALLENGED'
'and' is the link.
Can you help me to learn more?
(Other definitions for unchallenged that I've seen before include "not disputed" , "Not rivalled" .)