I found a 20 bit high side serial driver suitable to drive led's
it can receive data at 20 mhz when I only running a 8 Mhz clock
Do have to driver it with my software
as thier is no hardware support

and can i use a resister to slow the clock signal to simplify my software

 

We are going to need a bit more information to properly help with this - which MCU, which LED driver, links to datasheets if possible.
You will have to write some code to do it, but the amount depends on the serial capabiities of the MCU.
It's quite possible that 20MHz is the maximum speed and that the driver will run at less than that.