A river in Slavonia (4)

I believe the answer is:

avon

'a river' is the definition.
(I've seen this before)

'in slavonia' is the wordplay.
'in' indicates a hidden word.
'AVON' can be found hidden inside '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" .)

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