Genius teacher, mark you (10)
I believe the answer is:
mastermind
'genius' is the definition.
(I know that genius can be written as mastermind)
'teacher mark you' is the wordplay.
'teacher' becomes 'master' (I've seen this before).
'mark you' becomes 'mind' (I can't explain this - if you can you should give a lot more credence to this answer).
'master'+'mind'='MASTERMIND'
Can you help me to learn more?
(Other definitions for mastermind that I've seen before include "Intellectual" , "Be behind" , "brainbox" , "TV knowledge contest" , "Criminal planner" .)