Stick with New Testament disciple (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' means one lot of letters go next to another.
'new testament' becomes 'nt'.
'adhere'+'nt'='ADHERENT'
(Other definitions for adherent that I've seen before include "One supports" , "One supporting" , "Supporter; sticking" , "Weep" , "Supporter, fan" .)