The classical spectral Turán problem is to determine the maximum spectral radius of an $\mathcal{F}$-free graph of order $n.$ Let $\mathcal{K}_{k}^{-}$ be the set of all unbalanced $K_k.$ In this paper, we focus on the spectral Turán problem of $\mathcal{K}_{k}^{-}$-free unbalanced signed graph for $k\geq5$. Moreover, we give an answer for $k=5$ and completely characterize the corresponding extremal signed graph.