Never use leftovers! If you are asking about using some Clindamycin now that you have left over from a previous prescription, no. Clindamycin is a kind of antibiotic which can become harmful after its expiration date, and is usually not a first choice for a uti. If you think you have a uti, please see your doctor or an urgent care for appropriate testing and treatment.
Yes you can. It is not a first line antibiotic for a UTI like Bactrim (same as generic trimethoprim-sulfa) or cipro (ciprofloxacin). Or Macrodantin if UTI is confined to the bladder. Clindamycin, in my opinion is slightly less effective that the others, more expensive and has worse possible side effects of bloody diarrhea or jaundice. However may be indicated if your UTI is resistant to the 3 more commonly used antibiotics.
Prefer that you dont. Clindamycin(cleocin) is a very powerful third level antibiotic that we use for serious infections such as pelvic or kidney infections. Moreover, for simple uti's it is substantial "overkill" and may encourage antibiotic resistance, so when it is needed for a serious infection it will not work.The cdc has asked md's to be frugal with unnecessary antibiotic use as we are running out of antibiotics.