Country formerly part of Yugoslavia (6)
(Other definitions for bosnia that I've seen before include "Balkan country" , "Balkan state" , "European nation" , "Southeast European country (shortened form)" , "Balkan region joined with Herzegovina" .)
'country formerly part of yugoslavia' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for serbia that I've seen before include "Raise B in Yugoslavia" , "European country (bordering on 1 across)" , "Braise (anag) - country" , "South-east European country" , "Belgrade's country" .)
(Other definitions for croatia that I've seen before include "Adriatic country" , "East European country" , "Balkan country, capital Zagreb" , "European republic, became independent in 1991" , "Republic in SE Europe, capital Zagreb" .)