My brother in law has a 2007 Ford Focus which has just come up with a fualt code

Here is what he just emailed me

I had an amber light apear on the dash, running the self diagnostic it returned an error 0900, investigating the code it says its the Clutch Actuator Circuit/open, what does this mean? and how much is it likely to cost to put right? and is it safe to drive? there are a few other signs that have occured the car feels like its shuddering in 1st and 2nd (this is very intermitent and has only happened twice on 2 fairly long journeys) and when sitting idle the revs are around 860 which is a little lower than it normally is

Anyone got any idea's?

Cheer's Brian