Teacher might disapprove when they get nothing right (5)
I believe the answer is:
tutor
'teacher' is the definition.
(I know that tutor is a type of teacher)
'they get nothing right' is the wordplay.
'they' becomes 'tut' (I can't explain this - if you can you should believe this answer much more).
'get' says to put letters next to each other.
'nothing' becomes 'o' (O resembles 0).
'right' becomes 'r' (common abbreviation).
'tut'+'o'+'r'='TUTOR'
'might disapprove when' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for tutor that I've seen before include "Personal coach" , "Mentor" , "Individual teacher" , "Master" , "School" .)