。
That's certainly one idea.
Literal
That [topic-は] certainly one-idea [copula-です].
A polite half-agreement. 一案 ('one idea, one proposal,' literally 'one plan') is a noun phrase used to politely acknowledge a suggestion without fully endorsing it. 確かに ('certainly, indeed') is an adverb of acknowledgment, often used to grant a point before offering reservations. The whole utterance is diplomatic — it's the Japanese version of 'that's one way to look at it.'