Smashing up Balkan fences (6)
I believe the answer is:
superb
'smashing' is the definition.
(thesaurus)
'up balkan fences' is the wordplay.
'balkan' becomes 'serb' (I've seen this in another clue).
'fences' means one lot of letters goes inside another (inserted letters are fenced off).
'up' going into 'serb' is 'SUPERB'.
(Other definitions for superb that I've seen before include "choice" , "Triumphantly effective" , "Terrific" , "Impressively splendid" , "Magnificent, quite splendid" .)