http://wiki.project1999.com/Swish%27...ric_SOLO_guide
Cleric can solo 1-50 just fine.
Won't be swift, but it's just as do-able as a wizard soloing 1-50 (which is also not swift). I'm doing ~1% (slightly more) every 7ish minutes on my cleric at spectres in feerrott at lvl 40. If I catch a clarity/c2 from an enchanter heading into hate I can do 2 specs in that time if I don't get many resists.
However, you're going to have a lot more luck finding parties then a wizard and so you won't have to worry about soloing as much.