Split asunder (5)
(Other definitions for in two that I've seen before include "Completely divided" , "divided up thus?" , "Asunder" , "simply split" , "apart" .)
(Other definitions for riven that I've seen before include "Split" , "Being cracked" , "Torn apart" .)
'split asunder' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for burst that I've seen before include "Split asunder" , "Explosion" , "Ruptured" , "Crack" , "Rope to catch cattle" .)