Discipline is lacking in follower (8)
I believe the answer is:
disciple
'follower' is the definition.
(I know that disciple is a type of follower)
'discipline is lacking in' is the wordplay.
'is lacking' indicates named letters should be taken away.
'discipline' with 'in' removed is 'DISCIPLE'.
(Other definitions for disciple that I've seen before include "Christian?" , "Mark, for example" , "James perhaps" , "Personal follower of Christ" , "Learner in school" .)