Great Head of Repton, but initially bitter (5)
I believe the answer is:
acerb
'bitter' is the definition.
(synonyms)
'great head of repton but initially' is the wordplay.
'great' becomes 'ace' (both can mean a talented person).
'head of' suggests taking the first letters.
'initially' says to take the initial letters.
The first letter of 'repton' is 'r'.
The first letter of 'but' is 'b'.
'ace'+'r'+'b'='ACERB'
(Other definitions for acerb that I've seen before include "tart" , "sharp" , "harsh and bitter" , "Sour, bitter" .)