。
Write the address clearly and accurately.
Literal
Mail-of recipient-name [topic-は] clearly accurately.
An elliptical instruction — the verb 書いてください ('please write') is implied but omitted, a common stylistic choice for posted instructions and signs. 宛名 ('addressee name, recipient's name') is built from 宛 (addressed to) + 名 (name). はっきり ('clearly') is a state mimetic (擬態語) for clarity. A compact notice-style sentence.