Novel structures of polyoxometalates can be obtained by confining the synthons within specific nanospaces. This capability is what the confined synthetic method excels at. Within the cavity of {P₈W₄₈}, a novel multicomponent cluster comprising cationic Fe^III and Ce^III heterometals and PO₄³⁻ oxyanions was successfully nucleated for the first time. Alongside examining the structure and composition of this host–guest assembly, thorough investigations were conducted into the enhanced peroxidase-like activity induced by the Fenton-active metallic species. Preliminary studies on a colorimetric sensor based on [{Fe^III₈Ce^III₄O₂(OH)₁₂(H₂O)₈(PO₄)₂}(P₈W₄₈O₁₈₄)]²⁶⁻ yielded promising results, demonstrating its ability to detect ascorbic acid with high sensitivity and specificity.