、。
If he happens to drop by, tell him I'm busy.
Literal
By-chance he [subject-が] visit-come-[conditional-たら], him [to-に] I [topic-は] busy [quotative-と] say-[command-くれ].
訪ねてくる is the compound verb 訪ねる + 来る ('to come visiting'). くれ is the blunt imperative of くれる ('give to me') — here in its auxiliary sense, meaning 'do X for me.' The combination 言ってくれ is a rough, casual 'tell me / tell him for me.' The quoted content is framed by と.