We have synthesized a new spin-1/2 antiferromagnet, Y2Cu7(TeO3)(6)Cl-6(OH)(2), via a traditional hydrothermal method. This compound crystallizes in the triclinic crystal system with space group P (1) over bar. The magnetic ions constitute a two-dimensional layered lattice with a novel topological structure in which the Cu-4 clusters make up distorted diamond chains along the a axis and these chains are connected by the Cu-3 trimers. The magnetic susceptibility and specific heat measurements show that the compound is antiferromagnetically ordered at T-N = 4.1 K. This anti ferromagnetic ordering is further supported by electron spin resonance (ESR) data. The magnetization curve presents a field induced metamagnetic transition at 0.2 T, followed by a magnetization plateau within a wide magnetic field range from 7 T to at least 55 T, which corresponds to 3/7 of the saturated magnetization with g = 2.15 obtained from ESR. The possible mechanism for the magnetization plateau is discussed.