Follower's accepted in train (10)
I believe the answer is:
discipline
'train' is the definition.
(synonyms)
'follower's accepted in' is the wordplay.
'follower' becomes 'disciple' (disciple is a kind of follower).
'accepted' indicates putting letters inside.
'disciple' enclosing 'in' is 'DISCIPLINE'.
(Other definitions for discipline that I've seen before include "Correct" , "Punishment" , "Branch of knowledge; punish" , "Training designed to engender self-control" , "Correction" .)