Wicked pupils showing poor behaviour (3,4)
I believe the answer is:
bad form
'behaviour' is the definition.
(I have seen 'Unacceptable behaviour' mean 'bad form' so perhaps 'behaviour' could also mean 'bad form')
'wicked pupils showing poor' is the wordplay.
I cannot really see how this works, but
'wicked' could be 'bad' (similar in meaning) and 'bad' is located in the answer.
The remaining letters 'form' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bad form that I've seen before include "Offending against conventions" , "Unacceptable behaviour" , "Reprehensible conduct" , "unruly class" , "Breach of social conventions" .)