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