He's not the first to be 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 located in the answer.
'first' could be 'low' (synonyms) and 'low' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'to be' is the link.
Can you help me to learn more?
(Other definitions for follower that I've seen before include "Supporter" , "fan" , "Attendant" , "A loyal adherent - or a pursuer" , "One accepts the leadership of another" .)