Disciple: stick with New Testament (8)
I believe the answer is:
adherent
'disciple' is the definition.
(I know that disciple can be written as adherent)
'stick with new testament' is the wordplay.
'stick' becomes 'adhere' (to adhere to something is to stick to it).
'with' is a charade indicator (letters next to each other).
'new testament' becomes 'nt'.
'adhere'+'nt'='ADHERENT'
(Other definitions for adherent that I've seen before include "Staunch follower" , "supporter of party" , "Supporter; sticking" , "Sticking; partisan" , "Loyal follower - sticks close" .)