A river in Slavonia (4)
I believe the answer is:
avon
'a river' is the definition.
(I know that Avon is a river)
'in slavonia' is the wordplay.
'in' indicates the answer is hidden within the clue.
'AVON' is hidden in the letters of 'slavonia'.
(Other definitions for avon that I've seen before include "Stratford's river" , "Shakespeare its swan" , "[WILTS] flower" , "Shakespeare was known as the Bard of . . . ." , "runner" .)