River in Slavonic (4)
I believe the answer is:
avon
'river' is the definition.
(I know that Avon is a river)
'in slavonic' is the wordplay.
'in' indicates a hidden word.
'AVON' is hidden in the letters of 'slavonic'.
(Other definitions for avon that I've seen before include "River of the Bard" , "runner" , "Shakespeare its swan" , "Shakespeare was known as the Bard of . . . ." , "River at Shakespeare's Warwickshire birthplace" .)