Linear and nonlinear dielectric measurements under dc electric fields have been performed in an antiferroelectric liquid crystal MHPOCBC to obtain a deeper insight of dynamic properties related to a field-induced phase, designated as SmCX*, appearing when subjected to electric fields in the SmC alpha* phase. We observed a tricritical point in the SmC alpha*-SmCX* phase transition. We found two low frequency modes coming from domain walls in the first-order phase transitions to SmCX* and to SmC. In the third-order nonlinear dielectric measurements another low frequency mode was observed, of which frequency decreases near the SmC alpha*-SmCX* transition point. This may be the soft mode responsible for the field-induced transition.