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' (ace can informally mean very good).
'head of' says to take the initial letters.
'initially' says to take the initial letters.
The first letter of 'repton' is 'r'.
The initial 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" .)