He's not the first to be an adherent (8)
I believe the answer is:
follower
'adherent' is the definition.
(I've seen this before)
'he's not the first' is the wordplay.
I cannot quite understand 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 may be the basis of the clue (or it may be nonsense).
'to be an' acts as a 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" .)