Believer in training, not at home (8)
I believe the answer is:
disciple
'believer' is the definition.
(I know that disciple is a type of Christian)
'training not at home' is the wordplay.
'training' becomes 'discipline' (discipline is a kind of training).
'not' indicates named letters should be taken away.
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'discipline' with 'in' taken away is 'DISCIPLE'.
'in' is the link.
(Other definitions for disciple that I've seen before include "Devout follower" , "Student - follower" , "Follower of certain doctrines" , "Pupil - follower" , "A follower or apostle" .)