Holding back all those left outside school (9)
I believe the answer is:
restraint
'holding back all' is the definition.
I can't judge whether this defines the answer.
'those left outside school' is the wordplay.
'those left' becomes 'rest' (I've seen this in other clues).
'outside' is an insertion indicator (some letters go outside others).
'school' becomes 'train' (school can mean to train or coach).
'rest' enclosing 'train' is 'RESTRAINT'.
Can you help me to learn more?
(Other definitions for restraint that I've seen before include "Self-discipline" , "Something that holds back" , "Reserve" , "Curb" , "It keeps someone or something under control" .)