Boy back in detention at school (4)
I believe the answer is:
Here is my best explanation:
'boy' is the definition.
'back in detention at school' is the wordplay.
'back' says the letters should be written in reverse.
'in' says the answer is hidden in the clue.
'nats' can be found hidden inside 'detention at school'.
'nats' written backwards gives 'STAN'.