Break outside school, as a sort of discipline (9)
I believe the answer is:
restraint
'a sort of discipline' is the definition.
'break outside school' is the wordplay.
'break' becomes 'rest' (rest is a kind of break).
'outside' is an insertion indicator.
'school' becomes 'train' (training is a kind of schooling).
'rest' going around 'train' is 'RESTRAINT'.
'as' is the link.
(Other definitions for restraint that I've seen before include "It keeps someone or something under control" , "Start rain as an impediment" , "Reserve" , "Curb" , "controlling influence" .)