One teacher is wrong (5)
I believe the answer is:
amiss
'is wrong' is the definition.
('amiss' can be a synonym of 'wrong')
'one teacher' is the wordplay.
'one' becomes 'a'.
'teacher' becomes 'miss' (I've seen this before).
'a'+'miss'='AMISS'
(Other definitions for amiss that I've seen before include "Wrong; inaccurate" , "error" , "Wrongly" , "In an inappropriate way" , "Faulty" .)