Follower lacking in discipline (8)
I believe the answer is:
disciple
'follower' is the definition.
(disciple is a kind of follower)
'lacking in discipline' is the wordplay.
'lacking' indicates named letters should be taken away.
'discipline' with 'in' taken out is 'DISCIPLE'.
(Other definitions for disciple that I've seen before include "Adherent who helps to spread the doctrine" , "James perhaps" , "Follower of certain doctrines" , "Mark, for example" , "A faithful follower, as of Christ (8)" .)