Disciple to stick with the New Testament (8)
I believe the answer is:
adherent
'disciple' is the definition.
(I know that disciple can be written as adherent)
'stick with the new testament' is the wordplay.
'stick' becomes 'adhere' (to adhere to something is to stick to it).
'with' says to put letters next to each other.
'the new testament' becomes 'nt'.
'adhere'+'nt'='ADHERENT'
'to' acts as a link.
(Other definitions for adherent that I've seen before include "Supporter, fan" , "One believes in and helps spread the doctrine of another" , "Disciple, follower" , "Supporter, follower" , "One supports" .)