Adherent who helps to spread a doctrine (8)
I believe the answer is:
disciple
'adherent who helps to spread a doctrine' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for disciple that I've seen before include "A follower or apostle" , "Pupil - follower" , "Personal follower of Christ" , "Christian?" , "one believes" .)