Discipline in class (5)
I believe the answer is:
order
This clue is a double definition.
'discipline' is the first definition.
(I've seen this before)
'class' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for order that I've seen before include "Religious fraternity" , "Tidy state - that's a command" , "Issue a command" , "Group living under a religious rule" , "See 20" .)