In class this boy when ordered sat, the creep (8,3)
I believe the answer is:
teachers pet
'in class this boy' is the definition.
(term for a very studious child)
'ordered sat the creep' is the wordplay.
'ordered' indicates anagramming the letters.
'sat'+'the'+'creep'='satthecreep'
'satthecreep' with letters rearranged gives 'TEACHERS PET'.
'when' is the link.
(Other definitions for teachers pet that I've seen before include "Favoured pupil" , "Least popular child in the class?" , "Class favourite" , "Subject of favouritism at school" , "Student of form favoured" .)