Boy back in detention at school (4)
I believe the answer is:
stan
'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'.
(Other definitions for stan that I've seen before include "Laurel perhaps" , "- Getz, jazz musician" , "Eminem hit; man's name" , "- Laurel, Hardy's partner" , "- Laurel, comedian" .)