Eastern European (4)
'eastern european' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for serb that I've seen before include "One from Belgrade" , "Someone from Belgrade?" , "A European national" , "Eg, native of Belgrade" , "Balkan native" .)
(Other definitions for russian that I've seen before include "Native of Moscow, say" , "One from Petersburg" , "Slavonic tongue" , "Native of St Petersbourg, say" , "Slavic language" .)