。
Could you keep this a secret for me?
Literal
Secret [into-に] keep [in advance-ておいて] [for me-くれないか] [rhetorical-ね].
~ておいてくれないかね is a layered request: ~ておく (do in advance for later), ~てくれる (do for my benefit), ~ないか (won't you?), ね (seeking soft agreement). A polite-casual way to ask a favor, with a slightly older-male tone.