This question is in regards to a configuration where I want to setup a half-duplex SPI bus, with a master outputting a clock at 1MHz.
Does the slave need to know that the clock coming in will be 1MHz?
No, it doesn't. SPI is a synchronous protocol. That means that you transmit the clock as a discrete signal and it is used directly by the receiver to time the reception of the data. The receiver will have a maximum frequency at which it can operate, and some (such as some ADCs) will have a minimum frequency to maintain internal accuracy.