Chris:
Clindamycin is notorious for causing diarrhea, which as several others have pointed out is what is causing the high INR.
I've seen the diarrhea problems in cats when they've been RXed clindamycin after dentals/extractions. I usually stop the clindamycin and go to another antibiotic. I haven't used it systemically myself, only topically.
Lisou:
Clindamycin is RXed for treating anerobic bacteria -- bacteria that don't need oxygen to thrive. It probably wouldn't be RXed for a surface wound, unless it because a deep one.
My guess is clindamycin was RXed to treat any possible bacteria at the extraction site.