Serve Republican British bitter (5)
I believe the answer is:
acerb
'bitter' is the definition.
(synonyms)
'serve republican british' is the wordplay.
'serve' becomes 'ace' (type of tennis serve).
'republican' becomes 'r' (abbreviation).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'ace'+'r'+'b'='ACERB'
(Other definitions for acerb that I've seen before include "sharp" , "tart" , "Sour, bitter" , "harsh and bitter" .)