German master in German school - it's where you'd expect to see one (7)
I believe the answer is:
herring
'where you'd expect to see one' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'german master in german school' is the wordplay.
'german' becomes 'herr' (German equivalent to Mr).
'master in german school' becomes 'ing' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'herr'+'ing'='HERRING'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for herring that I've seen before include "Fish, often kippered" , "Fish that's kippered" , "Common food fish" , "Fish that can be red and kippered!" , "Small silvery sea-fish" .)