Prophet is first-class, given a head start (6)
I believe the answer is:
isaiah
'prophet' is the definition.
(I've seen this before)
'is first-class given a head start' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'given' is a charade indicator (letters next to each other).
'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 "A philosophical Berlin" , "Book of prophecies in the Bible" , "Book and prophet of the Old Testament" , "Sir - Berlin, twentieth century political historian" , "Biblical prophet (and his book)" .)