European capitals in Sweden, Estonia, Russia and Belgium (4)
I believe the answer is:
serb
'european' is the definition.
(I've seen this before)
'capitals in sweden estonia russia and belgium' is the wordplay.
'capitals in' indicates taking the first letters (I've seen 'capital in' mean this).
'and' says to put letters next to each other.
'belgium' becomes 'b' (international vehicle registration code).
The initial letters of 'sweden estonia russia' is 'ser'.
'ser'+'b'='SERB'
(Other definitions for serb that I've seen before include "Someone from Belgrade?" , "Balkan native" , "Eg, native of Belgrade" , "One from Belgrade" , "Eastern European" .)