Broadband dielectric spectroscopy was applied to investigate a newly synthesized antiferroelectric liquid crystal, (S)-(+)-4 '-(1-methylheptyloxycarbonyl)biphenyl-4-yl 4-(6-heptanoyloxyhex-1- oxy) benzoate. Near the SmA-SmC* phase transition the soft and the Goldstone modes are observed. The temperature dependence of the soft mode dielectric strength follows the Cure-Weiss law. In a narrow range of temperature the collective and antiferroelectric relaxation modes coexist as a result of the competition between ferroelectric and antiferroelectric order in the successive smectic layers. In the antiferroelectric SmCa* phase the low frequency, P-L, and high frequency, P-H, modes appear, which can be related to the in-phase and anti-phase azimuthal angle fluctuations of the director, respectively. The relaxation frequency of the PH and PL modes decreases linearly with decreasing temperature and satisfies the Arrhenius law. Some parameters involved in the extended Landau model of the SmA-SmC* phase transition could be obtained.