Teacher is wrong on a third of basic subjects (3)
I believe the answer is:
sir
'teacher' is the definition.
(way of addressing a male teacher)
'is wrong on a third of basic subjects' is the wordplay.
'wrong' is an anagram indicator.
'on' means one lot of letters go next to another.
'a third of basic subjects' becomes 'r' (I can't justify this - if you can you should believe this answer much more).
'is' with letters rearranged gives 'si'.
'si'+'r'='SIR'
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Polite term of address for a man" , "Title given to 10 Across" , "Honorary male title" , "Feted fellow" , "Baronet's title" .)