Clindamycin - Is it for sexual transmitted diseases?

Clindamycin is used for infections that are bacterial in nature. So it really depends on the cause of the sexually transmitted disease. It is an alternative to use when people are allergic to penicillin. Uses include as quoted from drugs.com "Serious infections caused by susceptible organisms:
-Staphylococci, streptococci, and pneumococci, including serious respiratory tract infections and skin and soft tissue infections
-Anaerobes, including serious respiratory tract infections, serious skin and soft tissue infections, septicemia, intraabdominal infections, and infections of the female pelvis and genital tract.
-Bone and joint infections including acute hematogenous osteomyelitis caused by Staphylococcus aureus and as adjunctive therapy in the surgical treatment of chronic bone and joint infections.

So yes it can be used for some sexually transmitted diseases, but not all.