Large nonlinear effects are found in a well-defined temperature range above the Neel point in a disordered antiferroelectric. The sign of the nonlinear susceptibility is opposite to that expected for clusters with internal ferroelectric interactions, indicating that the effect involves antiferroelectric clusters. An extended regime of third-harmonic generation which looks nonanalytic as a function of ac bias amplitude down to small fields is found, indicating that the distribution of cluster sizes extends to very large values, as expected for a Griffiths phase. However, the dependence of the susceptibility on dc fields shows no nonanalyticity, indicating that there is no true Griffiths phase.