I went through this about 8 years ago. Same deal as you, it was a back molar that had a root canal done at one point and the pain came on suddenly and painfully! I also had some swelling in my face. I went to my dentist on a Friday after 2 days of mild pain, who put me on an antibiotic and some pain killer that didn't work for anything. I was in AGONY over the weekend while waiting for my Mon. appt. and to clueless at the time to call the Endodontist's emergency #. I literally would give birth any day over having to go through that pain again. I don't know what Clindamycin is but I hope it's working for you!
As far as the procedure, everything was fine! It was about 2 hours. I went into the office in tears from the pain, they gave me novacaine and I felt no pain during the procedure. They go into the tooth and drain out all of the infection, which I guess oozes out of the tooth. I had no pain by the end of my first appt. but did have to go back 2 additional times for them to completely get all of the infection out. It's important for them to do that before sealing the tooth back up. I was on an antibiotic and painkiller after, but I really didn't feel I needed the painkiller at that point.
I would not stress about the appt. at all. It's going to be fine! The appt. was the easiest part of the whole ordeal for me because it relieved me of the pain. And, it is so worth seeing the Endodontist because you don't want to just have the tooth pulled, like my idiot dentist recommended at the time as a quick fix, I'm glad I had better sense. Good luck!