A 45-year-old man presents with a 3-day history of an erythematous, warm, tender area on his lower leg with poorly defined margins. He is febrile to 38.2 °C but haemodynamically stable and has no systemic toxicity. There is no purulence or abscess and he has no penicillin allergy. According to Australian antibiotic guidelines, which oral agent is most appropriate first-line for this mild non-purulent cellulitis?