The RF power production efficiency is one of the major concerns in the 3-TeV Compact Linear Collider (CLIC) Drive Beam Accelerator, as it is responsible for almost 50% of the entire accelerator complex power consumption. The klystron technology with dc post acceleration of the prebunched beam was reconsidered recently, as an attempt to develop a technological solution for the high efficiency, high power {L} -band multibeam klystrons (MBKs). In this article, the modeling and design study of the CLIC MBK with two-stage (TS) configuration is presented. The CLIC TS MBK performance validation has been done using the sophisticated 3-D particle-in-cell computer simulations of the entire device.