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 in another clue)
'he's not the first' is the wordplay.
I cannot quite see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'first' could be 'low' (synonyms) and 'low' is present in the answer.
This explanation may well be incorrect...
'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" .)