According to Fleming it’s M for misbehaviour? (8)
I believe the answer is:
mischief
'misbehaviour?' is the definition.
(mischief is a kind of misbehaviour)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for mischief that I've seen before include "injury" , "Monkey business" , "nuisance" , "(Playfully) destructive behaviour" , "bad behaviour" .)