Couple of peripherals like SPI and UART derive their clock from the fPCLK. So what clock is PCLK and what frequency is fPCLK?

PCLKx is the clock of the corresponding APB peripheral X. For example:

Clock Bus
PCLK1 APB1
PCLK2 APB2

Note: Similarly, HCLKx is the clock of the corresponding AHB peripheral X. For example:

Clock Bus
HCLK1 AHB1
HCLK2 AHB2

So, when SPI2 says it derives its clock for fPCLK, what it means is the clock of its APB bus. In STM32F407, SPI2 is on APB1. So this makes its fPCLK fPCLK1.