In river battle, he destroyed tyrant (4,3,8)
I believe the answer is:
ivan the terrible
'tyrant' is the definition.
(I've seen this before)
'in river battle he destroyed' is the wordplay.
'destroyed' indicates anagramming the letters.
'in'+'river'+'battle'+'he'='inriverbattlehe'
'inriverbattlehe' is an anagram of 'IVAN THE TERRIBLE'.
(Other definitions for ivan the terrible that I've seen before include "Dictator" , "Russian tsar" , "Dreadful ruler" , "First tsar of all Russia" , "Cruel tyrant" .)