Can I take Mucinex (guaifenesin) for sinusitis? If not is there any other over-the-counter medicine alternative to clindomycin?

They're different. Clindamycin is an antibiotic that is not commonly used for initial treatment of sinusitis. Mucinex (guaifenesin) is a medication that may help you because it loosens thick mucus. Also try a decongestant like pseudoephedrine (e.g., Sudafed) and saline sinus rinses for best results. It is not recommended to start antibiotics for sinusitis unless not improving after 7 days.