Excellent Balkan citizen keeps winning (6)
I believe the answer is:
superb
'excellent' is the definition.
(I've seen this before)
'balkan citizen keeps winning' is the wordplay.
'balkan citizen' becomes 'serb' (I've seen this in another clue).
'keeps' indicates putting letters inside.
'winning' becomes 'up' (I've seen this in other clues).
'serb' placed around 'up' is 'SUPERB'.
(Other definitions for superb that I've seen before include "Impressively splendid" , "Dusty product of spider" , "fine" , "Magnificent, quite splendid" , "First-rate" .)