Very easy. Go into the controler settings in LFS, and go to the AXIS tab. Also, before you do anything else, change the clutch to an Axis, not a button (to change it, it should in the up'er part of the screen). From there, hit "Clutch" (should be close to the bottom of the screen), then match it up to the Axis that your brake pedel is on (you might need to invert that axis though).
I hope that was kind of clear enough to help you out. If you need more help on this, just shoot me a pm, and i'll try to make it more clear.