Even a school mistress can be wrong! (5)
I believe the answer is:
amiss
'be wrong' is the definition.
(synonyms)
'even a school mistress' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
'school' could be 's' and 's' is found within the answer.
The remaining letters 'mis' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'can' is the link.
Can you help me to learn more?
(Other definitions for amiss that I've seen before include "Unsuitable" , "Wrongly" , "astray" , "In an inappropriate way" , "Faulty" .)