$\{ww^R\mid w\in\{a,b\}^∗\}$ should do the job. The rest is up to you regarding proofs, such as providing an unambiguous grammar.
From Example of Non-Linear, UnAmbiguous and Non-Deterministic CFL? Third answer by Google.
Another one found similarly at Non-Deterministic CFLs: $\{x^ny^n \;\mid\; n \geq 0\} \cup \{x^ny^{2n} \;\mid\; n \geq 0\}$
