first it seems like Clayman wins against rimuru but it's just an illusion shown to him by Rimuru. In the illusion events unfold like Wisdom king Raphael predicts clayman planned it. Then rimuru just completely wrecks him.
@Metasepia : I don't read the LN, so this is just my guess.
Either:
A) There's a backup binding to prevent his soul from being freed so Clayman can 'revive' him even if he falls in battle.
B) After taking over the domain, Shuna erect a barrier that ends up trapping their souls inside, and Rimuru decide to revive them later on.