He's not the first to become an adherent (8)
I believe the answer is:
follower
'an adherent' is the definition.
(I've seen this before)
'he's not the first' is the wordplay.
I cannot really see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'first' could be 'low' (synonyms) and 'low' is found within the answer.
This may be the basis of clue (or it may be nonsense).
'to become' acts as a link.
Can you help me to learn more?
(Other definitions for follower that I've seen before include "Supporter" , "A loyal adherent - or a pursuer" , "A devotee or a pursuer" , "Person who succeeds" , "Attendant" .)