At class, a pupil will be on best behaviour (6)
I believe the answer is:
formal
'at class a pupil will be on best behaviour' is the definition.
I can't judge whether this definition defines the answer.
'at class a pupil' is the wordplay.
'at' says to put letters next to each other.
'class' becomes 'form' (form class is a kind of class).
'pupil' becomes 'L' (as in L-plates for a learner driver).
'form'+'a' is 'forma'.
'forma'+'l'='FORMAL'
Can you help me to learn more?
(Other definitions for formal that I've seen before include "stuffy" , "Conforming to accepted rules, as of dress" , "conventionally correct" , "Precise - ceremonial" , "Essential" .)