^Seconded^.
Penetrating oil, aka PB Blaster (or just Blaster), Liquid Wrench and some new stuff made by the WD-40 people called "Pro" something (but not actual WD-40 which is dang near useless) are some of the best penetrants. Douse it liberally like Anickode said.
Then you're gonna (most likely) need some persuasion with a hammer of decent size and weight. A ball peen hammer is what you need, with which you then you rap with (like at 12, 6, 3 and 9 o'clock) around the circumference of the clutch piece and see if you can knock it loose. If not immediately, then at least you're inducing vibrations in the pieces that can shake them loose or dislodge some rust particles.
Heat (propane torch), brute force and more penetrating fluid are your friends here. My fervent hope is that you are successful in slaying this demon. Good luck!
EDIT--the rusted outer portion of the shaft is a devious ally in league with the rusted clutch.
It appears you will have to polish the shaft back to shiny brightness and smoothness (or close to it) in order for the clutch to move past it.
