A fiery one, style not unknown, but one of the faithful (7)
I believe the answer is:
apostle
'one of the faithful' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'a fiery one style not unknown' is the wordplay.
'a fiery one' becomes 'apo' (I can't justify this - if you can you should give a lot more credence to this answer).
'not' is a deletion indicator.
'unknown' becomes 'y' (algebra).
'style' with 'y' taken away is 'stle'.
'apo'+'stle'='APOSTLE'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for apostle that I've seen before include "Divine messenger" , "A follower of Jesus" , "[JAMES] , for one?" , "James, perhaps" , "One of the biblical twelve (7)well don" .)