Prophet is first-class given a head start (6)
I believe the answer is:
isaiah
'prophet' is the definition.
(I know that Isaiah is a prophet)
'is first-class given a head start' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'given' means one lot of letters go next to another.
'start' says to take the initial letters.
The initial letter of 'head' is 'h'.
'is'+'ai'+'a'+'h'='ISAIAH'
(Other definitions for isaiah that I've seen before include "Biblical Book of Prophecies" , "After Psalms, the longest book in the Old Testament" , "A philosophical Berlin" , "8th century Hebrew prophet and O.T. Book" , "Book of prophecies in the Bible" .)