Clindamycin is an antibiotic used to treat bacterial infections. To diagnose your cat with a urinary bacterial infection, I'm guessing they obtained a sample to look at under the microscope? If they were able to actually see which bacteria was causing the problem then that makes it easier to choose which antibiotic would work best. While some meds may not be the most commonly used for some things, if the vets previous experiences with drug A to treat infection B then they tend to stick with what works. Sometimes that means 'off lable' use of a drug, a use that the drug is not specifically aimed at treating if that is what has worked in their experience.

If you are concerned that you have the right meds then I'd call the office tomorrow morning and ask to be sure so you have peace of mind.

Clindamycin is a bitter tasting medicine, especially as a liquid. I hope your vet flavored it for you. If not and your cat is really disliking the meds then ask your vet if he can flavor it for you.

I hope the infection goes away soon.